Rails 4 version fix (comparison of Gem::Version with String failed)

Rails.version in Rails 4 will return Gem::Version.new not String. 

This will raise exception:

/Users/madmax/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/bundler/gems/bootstrap-datepicker-rails-4ea02449857e/lib/bootstrap-datepicker-rails.rb:6:in `<': comparison of Gem::Version with String failed (ArgumentError)