Installing pg gem for deploying rails app via heroku (windows)

Running 'heroku run rails console' I get the following error:

Running `rails console` attached to terminal... up, run.7453
    /app/vendor/bundle/ruby/2.1.0/gems/activerecord4.2.1/lib/active_record/connection_adapters/connection_specification.rb:177:in
 `rescue in spec': Specified 'postgresql' for database adapter, but the gem is
 not loaded. Add `gem 'pg'` to your Gemfile (and ensure its version is at the
 minimum required by ActiveRecord). (Gem::LoadError)

This is in my Gemfile :

gem 'pg', '~> 0.18.1'