travis: Downgrade to previous images temporarily
Travis is in the process of [rolling out an update][update] but looks like our
tests are breaking, let's temporarily roll back to get the queue moving again.
[update]: https://blog.travis-ci.com/2017-08-29-trusty-image-updates
language: shell
sudo: required
dist: trusty
+# FIXME(#44398) shouldn't need to be here
+group: deprecated-2017Q3
services:
- docker