Speed up the multi-stage Travis run (#443)

Reduces the impact on Travis runtime caused by #438, by:
* Making the shellcheck job run on the container infra (`sudo: false`)
  which has faster boot times.
* Running the heroku-16 and cedar-14 stack tests in parallel, since
  previously they were separate stages (which are run in series by
  design).

Reduces Travis end-to-end time from ~10 minutes to ~6 minutes.