Don't test Gerrit on Ubuntu 14.04
It's broken, because it requires Git 2.2, which is not available on that version of Ubuntu:
---
platforms:
- ubuntu1404:
- build_targets:
- - "//:release"
- test_flags:
- - "--test_tag_filters=-slow"
- test_targets:
- - "..."
ubuntu1604:
build_targets:
- "//:release"
