Faster Travis CI builds

[DeeDeeG]

Context

• Implements ideas laid out in this comment:
  • https://github.com/RefugeRestrooms/refugerestrooms/pull/586#issuecomment-493680534
  • Credit goes to @btyy77c for essentially doing the same thing in the dockerAlpine branch on their fork. I am just re-doing the same optimizations in a way appropriate for our develop branch at this time. (And without changing the main image to be Alpine-based, at least for now.)
• Makes builds go faster at Travis-ci.com without negatively impacting development (and without any impact on production whatsoever, since we don't use Docker or Travis CI in production).

Summary of Changes

• Use the minimal language for Travis CI, instead of the ruby language.

• Use the smaller postgresql:alpine base image for our Docker database container, instead of the generic postgresql image, which is based on debian.

  Checklist

• [ ] Tested Mobile Responsiveness

• [ ] Added Unit Tests

• [ ] CI Passes

• [ ] Deploys to Heroku on test Correctly (Maintainers will handle)

• [ ] Added Documentation (Service and Code when required)

[DeeDeeG]

Would also like to merge this PR this weekend.

- DeeDeeG

[DeeDeeG]

Also merging this today.