Specs: Don't include Deterministic in global scope. Fixes #12

I noticed a bug when playing around with mixing Deterministic into a class. Then I looked at the tests, and noticed that mixing into a class wasn't being tested in an isolated context, due to Deterministic being included into the global context.

This PR scopes all includes into `describe` blocks, avoiding the global context. With this, the bug I discovered in #12 was properly exposed, so I fixed that too.