change usage of allow() to expect()

Yet another :rabbit2: hole: was running into issues where I was trying to refactor (in https://github.com/18F/C2/pull/127), but it was unclear whether the mocks were still valid. `allow` doesn't give any warning if that method is never called, so it wasn't clear what I had broken, vs. what weren't applicable in the first place. Let me know if that doesn't make sense :smile: