Cache ansible galaxy roles in Travis CI

This should considerably speed up CI builds and we rarely touch add or
remove vendor roles. If that was the case, Travis will store any
modifications in the cache. I guessed that from docs and a PR:

> Only modifications made to the cached directories from normal pushes
are stored.

source: https://docs.travis-ci.com/user/caching

> If nothing in the dist directory changed during the build, nothing new
will be uploaded.

source:
travis-ci/travis-ci#9814 (comment)

.travis.yml

@@ -3,6 +3,10 @@ dist: bionic
 language: python
 python: 3.7.4
 
+cache:
+  directories:
+    - vendor
+
 jobs:
   include:
     - stage: lint