Trying to update Rack gem to 1.5.4 for Rails 3.2 applications

Due to the Denial of Service vulnerability with the Rack gem, I am trying to update the Rack gem from version 1.4.5 to 1.5.4 but receive the below

Bundler could not find compatible versions for gem "rack":
In Gemfile:
rails (= 3.2.22) ruby depends on
  railties (= 3.2.22) ruby depends on
    actionpack (= 3.2.22) ruby depends on
      rack (~> 1.4.5) ruby
rack (~> 1.5.4) ruby

I am not sure what my options are for how to go about updating the rack gem to version 1.5.4 for Rails 3 applications. Should I be able to update the Rack gem to 1.5.4 for Rails 3 applications without any issues?

The way I went about updating the Rack gem within a Rails 3.2 application is: a) Deleted my Gemfile.lock, b) emptied application's gemset, b) Added within the application's Gemfile: gem 'rack', '~> 1.5.4', c) ran "bundle install"