Fix verify_commit_hooks test to expect 404 instead of 201

The verify_commit_hooks test was failing for the pre4 milestone.
This change fixes the test to expect a 404 (NOT FOUND) instead of a 201 (CREATED).

* Justification: riak_kv_wm_object:ensure_doc(Bucket,undefined) will return {error, undefined}, which will lead to a 404.
* See original fix to ensure_doc here: https://github.com/basho/riak_kv/pull/237