Test Heroku-16 and Cedar-14 in parallel on Travis

This reduces the wall-clock time from 16 minutes to 9 minutes.

.travis.yml

@@ -2,5 +2,8 @@ language: bash
 sudo: required
 services:
   - docker
-install: docker pull heroku/cedar:14
-script: make test
+env:
+  - STACK=heroku-16
+  - STACK=cedar-14
+install: docker pull "heroku/${STACK/-/:}"
+script: make test-${STACK}