Add #to_s method to rails version

```Rails.version``` now returns an instance of ```Gem::Version```
See https://github.com/rails/rails/pull/8501

This results in the following exception:

/Users/wouterw/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/bundler/gems/underscore-rails-6e01769de4b2/lib/underscore-rails.rb:5:in `>=': comparison of Gem::Version with String failed (ArgumentError)