Fix deprecation warning about constant usage

Rails 5 requires looking up registered mime types based on their defined
symbol aliases:

> DEPRECATION WARNING: Accessing mime types via constants is deprecated.
> Please change `Mime::JSON` to `Mime[:json]`. (called from formats at
> lib/kracken/json_api/request.rb:53)

Author: cupakromer

lib/kracken/json_api/request.rb

@@ -48,9 +48,9 @@ def formats
             accepts
           # original_format would be set by the ABOVE two conditions
           elsif xhr?
-            [Mime::JS]
+            Mime[:js]
           else
-            [Mime::JSON]
+            Mime[:json]
           end
         end
       end