Add support for Gemfile.local.rb
Currently to tweak the bundler Gemfile, it makes it difficult to track the master repo, and everytime it is updated changes have to be merged. Seperating out customizations into a separate file would largely address this.
See the following for details on how to do this: http://stackoverflow.com/questions/4244182/how-to-customize-gemfile-per-developer
We need to add Gemfile.local.rb to .gitignore as well.