Update Gemfile.heroku with 'twitter' gem dependency

In production.rb `config.cache_classes = true`, and when i'm trying to start the app, i have this error: `..../gems/activesupport-3.2.14/lib/active_support/dependencies.rb:317:in 'rescue in depend_on': No such file to load -- twitter (LoadError)`.

If i change true to false it's ok.

After a brief search i found that this is because of 'twitter' gem dependency, which is absent in Gemfile.heroku (which i used)