uninitialized constant ActiveModel::Observing (NameError)

I have developed a web application using ruby on rails. I have upgraded my rails version from 3.2 to 4.0 and I am getting the below error while starting my server.

Error :

/home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/dynamoid-0.6.1.1/lib/dynamoid/config.rb:11:in <module:Config>': uninitialized constant ActiveModel::Observing (NameError) from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/dynamoid-0.6.1.1/lib/dynamoid/config.rb:8:in' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/dynamoid-0.6.1.1/lib/dynamoid/config.rb:5:in <top (required)>' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/1.9.1/rubygems/custom_require.rb:36:inrequire' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/1.9.1/rubygems/custom_require.rb:36:in require' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/dynamoid-0.6.1.1/lib/dynamoid.rb:21:in' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:76:in require' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:76:inblock (2 levels) in require' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:74:in each' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:74:inblock in require' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:63:in each' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler/runtime.rb:63:inrequire' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/bundler-1.3.5/lib/bundler.rb:132:in require' from /home/lakhwani/FCLaunchRequirement/src/FCLaunchRequirementWebsite/rails-root/config/application.rb:88:in' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/1.9.1/rubygems/custom_require.rb:36:in require' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/1.9.1/rubygems/custom_require.rb:36:inrequire' from /home/lakhwani/FCLaunchRequirement/src/FCLaunchRequirementWebsite/rails-root/config/environment.rb:6:in <top (required)>' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/1.9.1/rubygems/custom_require.rb:36:inrequire' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/1.9.1/rubygems/custom_require.rb:36:in require' from /home/lakhwani/FCLaunchRequirement/src/FCLaunchRequirementWebsite/rails-root/config.ru:3:inblock in ' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/builder.rb:55:in instance_eval' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/builder.rb:55:ininitialize' from /home/lakhwani/FCLaunchRequirement/src/FCLaunchRequirementWebsite/rails-root/config.ru:in new' from /home/lakhwani/FCLaunchRequirement/src/FCLaunchRequirementWebsite/rails-root/config.ru:in' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/builder.rb:49:in eval' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/builder.rb:49:innew_from_string' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/builder.rb:40:in parse_file' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/server.rb:277:inbuild_app_and_options_from_config' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/server.rb:199:in app' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/server.rb:314:inwrapped_app' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/server.rb:250:in start' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/lib/rack/server.rb:141:instart' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/ruby1.9/gems/1.9.1/gems/rack-1.5.2/bin/rackup:4:in <top (required)>' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/bin/rackup:23:inload' from /home/lakhwani/FCLaunchRequirement/env/FCLaunchRequirementWebsite-1.0/runtime/bin/rackup:23:in <main>' /home/lakhwani/FCLaunchRequirement/env/BrazilRake-1.1/runtime/ruby2.1.x/lib/ruby/site_ruby/2.1.0/amazon/brazil/ruby.rb:98:inexec_build_script': Command exited with status 1!! (RuntimeError) from /home/lakhwani/FCLaunchRequirement/env/BrazilRake-1.1/runtime/bin/brazilrake:61:in block in <main>' from /home/lakhwani/FCLaunchRequirement/env/BrazilRake-1.1/runtime/bin/brazilrake:48:ineach' from /home/lakhwani/FCLaunchRequirement/env/BrazilRake-1.1/runtime/bin/brazilrake:48:in `'

Can any one help me resolve this issue?