Revert Rails-module hack and no longer expire Sprockets index in development

I realized that my hack to attempt to fix `jquery-rails` turns around and breaks `bootstrap-sass` which correctly detects for the presence of Rails. So I've removed my hack and the tests don't pass anymore. We should file a bug against jquery-rails instead.

The second commit fixes middleman/middleman#734 by no longer expiring the Sprockets index all the time. I couldn't find any situation where Middleman doesn't pick up changes to JS or CSS with this removed.