Rails 4 fix.

The functionality of cache_digests has been brought to Rails 4 with some changes.

One change from the cache_digests gem is the method you use to get a cache digest.
In Rails 4 you use `cache_fragment_name` while with cache_digests you call
`fragment_name_with_digest`.
Merged in upstream changes.
Merge pull request #20 from feedbin/rails_4

Rails 4 fix.