Cleanup check for html content type

[`Mime::Type.html?`][docs] is a publicly available API that internally
uses a regex.

[docs]: http://api.rubyonrails.org/classes/Mime/Type.html#method-i-html-3F

Author: wireframe

CHANGELOG.md

@@ -1,6 +1,10 @@
 master
 ------
 
+* Use `Mime::Type.html?`
+
+[#523]: https://github.com/thoughtbot/ember-cli-rails/pull/523
+
 0.8.4
 -----
 

lib/ember_cli/ember_constraint.rb

@@ -11,9 +11,7 @@ def rails_info_request?(request)
     end
 
     def html_request?(request)
-      index = request.format.to_s =~ /html/ || -1
-
-      index > -1
+      request.format.html?
     end
   end
 end