Slimmed down test dependencies.

* Removed Integration tests. I know this will be controversial. Lets add them
  back in when they don't massively slow down the entire suite.
* Use minitest/mock instead of Mocha. Again, happy to be swayed, but I think
  minimal dep footprint for Assembly apps is a good thing.
Update travis to run all tests as one command.
Stub out Webhook worker.