Add `herokuish buildpack test` command
Submitted to upstream https://github.com/gliderlabs/herokuish/pull/271. Once merged and released:
- we can close this MR
- switch to upstream herokuish images for zero-config CI
This MR is to keep track of changes we did for zero-config CI support. I will build our own image of herokuish
based on this changes so we can proceed with https://gitlab.com/gitlab-org/gitlab-ce/issues/26941.