Changed ruby declaration in Gemfile to comment directive.

When running bundle install using ruby-2.0.0-p247 I got an error 'Your Ruby
version is 2.0.0, but your Gemfile specified 2.0.0-p247', it seems that the
Gemfile ruby declaration cannot specify a version. To use more fine grained
version we can use a comment directive see https://rvm.io/workflow/projects.