travis: move back to the minimal image

The most recent update replaced the minimal image with a large one for the
'generic' image. Switching back to 'minimal' should reduce dependencies and
maybe speed us up some.

It should also eliminiate the need for aa2e0f09e.

Github-Pull: #11521
Rebased-From: a86e81b78f
Tree-SHA512: 40f07304c60b11876a172b7ac75cb738b2da4209d54370714a6e731485643fbb33925a422accd3e065dcaa4fea84a1268f37ca0a8ee83b8cf70a71a25b59fb6b

.travis.yml

@@ -1,7 +1,7 @@
 sudo: required
 dist: trusty
 os: linux
-language: generic
+language: minimal
 cache:
   directories:
   - depends/built