More dependency fixes.

Specifically:

1. ZenTest 4.8.4 is by fiat Not Broken, so the exclusion might as well
   be removed.
2. WebMock 1.9.2 expects Mutex to be defined (probably via thread) but
   that constant is not present when running on 1.8.7 (perhaps thread
   isn't being required).  Therefore, we lock to 1.9.0 for now, which
   although possibly buggy in a multithreaded context does at least work
   with Castanet's tests.