Fix failing tests by requiring mocha

Now that the rails gem points to 4.2, we need to require mocha in order
to use the `expects` syntax, otherwise tests will fail. See:
https://travis-ci.org/rails/actionpack-action_caching/jobs/46021642

test/caching_test.rb

@@ -1,4 +1,5 @@
 require 'abstract_unit'
+require 'mocha/setup'
 
 CACHE_DIR = 'test_cache'
 # Don't change '/../temp/' cavalierly or you might hose something you don't want hosed