.travis.yml: Use minimal base image for Travis CI

We do all the setup/build steps inside a Docker container,
so we don't need ruby tools outside of Docker
(on the Travis CI virtual machine instances).

Should save about 20 seconds of Travis CI build time.

--

Inspired by @btyy77c who did this first at their dockerAlpine branch:
  - btyy77c@393cf46

Documentation at Travis re: minimal/generic images:
  - https://docs.travis-ci.com/user/languages/minimal-and-generic/

Author: DeeDeeG

.travis.yml

@@ -1,7 +1,7 @@
 env:
   global:
     - CC_TEST_REPORTER_ID=c18df080592f9c99ca8080a6d5e052aa5fd3964044a0fe0b71e48f8e18998dc2
-language: ruby
+language: minimal
 services: docker
 install:
   - docker-compose build