Travis: Reduce end to end testing time (#1022)
Travis: Reduce end to end testing time (#1022)

This change (along with #1021, which skips an unnecessary docker build)
reduces the wall clock time from ~22 minutes to ~6 minutes. Even with
the additional overhead from increased parallelism, the combined job
duration (~50 minutes) has not increased due to the other time savings.

Changes:

- for the unit tests, each stack is now tested in its own job and so
  tested in parallel
- the use of Travis stages has been removed, since by design it blocks
  later tasks on earlier stages having completed - reducing parallelism
  unnecessarily for this use case
- all jobs except for the Hatchet job now use Travis' `minimal` image,
  and no longer install redundant Ruby + bundler
- the `sudo: {required,false}` references have been removed, since
  Travis no longer supports its non-sudo container infrastructure so
  ignores that option

Fixes #1018.

[skip changelog]
AltSchool/heroku-buildpack-python,Scalingo/python-buildpack,heroku/heroku-buildpack-python,AltSchool/heroku-buildpack-python,Scalingo/python-buildpack,skilljar/heroku-buildpack-python,heroku/heroku-buildpack-python