Stomp.py connection exit

Resolves #4.

I'm unable to reproduce the problem in #4, but the change has been tested and seems to work fine, so I'm happy to use `stop()`.

Main changes:

- Some tweaks to the Travis CI file.
- Change close_connection method from using `disconnect()` to `stop()` as the
  latter actually waits for the connection thread to exit after calling
  `disconnect()` itself.
- Reformat error message in test_crypto.