Update install generator to detect application.coffee to fix nil error on install

The install generator currently fails with a nil error 
if an application.coffee exists because its presence is not 
detected from #detect_js_format. Since the asset pipeline handles 
straight .coffee files, we should support this during the install 
generator.
Merge pull request #2157 from chrismccord/master

Fix nil error on rails install generator by detecting application.coffee