Fix webhook response in TravisCI integration, test failures.

Review Request #10655 — Created Aug. 7, 2019 and submitted — Latest diff uploaded


This change adds a missing HttpResponse return to the TravisCI
webhook. We didn't notice this before because by the time the error
gets triggered, everything has already been processed and updated, so
it's just something that triggers an error email to the server admins.

While I was doing this I found there were a handful of test failures.
Some args-vs-kwargs problems were causing a few tests to error out,
either with a duplicate kwarg error or a type error; these were easily
fixed by specifying kwarg names for all IntegrationForm
instantiations. There were also some warnings coming from kgb about
missing owner= parameters.

Ran unit tests.