Remove some instances of time.sleep
Remove more instances of time.sleep in tests
Always use a timeout when calling event.wait()

This stops the tests for hanging forever if an event doesn't happen.
If your computer is really slow it may cause the tests to fail due to a
race condition, but the timeout is big enough that this isn't a serious
worry.
Install requirements-dev.txt on Travis
Make _TEST_CALLBACK_URL a config setting

Make _TEST_CALLBACK_URL an optional config setting instead of a database
column. There's no need to store the same URL in every cell or a db
column anyway, and this avoids the need for a db migration, and takes
less code.
Merge branch 'seanh-remove-time-sleep'