.js.coffee -> .coffee

It was initially required, but support for the shorthand has been
supported since sprockets 2.1. Eventually 4.x will only support the
shorthand version. Just want to get new people using the prefer stuff
ASAP.

turbolinks.gemspec

@@ -8,7 +8,7 @@ Gem::Specification.new do |s|
   s.license  = 'MIT'
   s.homepage = 'https://github.com/rails/turbolinks/'
   s.summary  = 'Turbolinks makes following links in your web application faster (use with Rails Asset Pipeline)'
-  s.files    = Dir["lib/assets/javascripts/*.js.coffee", "lib/turbolinks.rb", "lib/turbolinks/*.rb", "README.md", "MIT-LICENSE", "test/*"]
+  s.files    = Dir["lib/assets/javascripts/*.coffee", "lib/turbolinks.rb", "lib/turbolinks/*.rb", "README.md", "MIT-LICENSE", "test/*"]
   s.test_files = Dir["test/*"]
 
   s.add_dependency 'coffee-rails'