Fix compatibility with Rails 4.1 by forward-porting a require statement to load FileSystemResolver

* Fixes #44
* I've transplanted this commit from netzpirat/haml_coffee_assets@7167b263e9bf852b2c63906752cb521f2ec89fc7
* This fixes mobylette-3.5/lib/mobylette/resolvers/chained_fallback_resolver.rb:3:in `<module:Resolvers>': uninitialized constant ActionView::FileSystemResolver (NameError)
* For more details, see rails @ 3fbff78 (rails/rails@3fbff78#diff-90f6f2b6859f5da65e11631ebcdf3f09)