Don't require Rails

Hi guys,

I work on https://github.com/middleman/middleman, and we use Sprockets to support JavaScript in a similar way to Rails. However, that can be weird when we use gems that provide JS to Sprockets without the Rails context.

Here's what causes the problem in jquery-rails:
https://github.com/rails/jquery-rails/blob/master/lib/jquery/rails.rb

Would it be possible for those lines to be conditional on the existence of the Rails module?