Fix specs

My bad. Seems like I added a commit on celluloid/celluloid which calls `Celluloid.shutdown` in one of the shared actor specs. This works there since we start Celluloid before each spec with a similar `around` filter to this one.

Isolating the global state between examples seems like a good idea anyway, imo.
Merge pull request #30 from jnicklas/fix-specs

Fix specs