Change .travis.yml's language: minimal to generic.

There is no `minimal` language. Due to travis-ci/travis-ci#4895, it will
fallback to `ruby`, which certainly isn't what we want. `generic` is an
undocumented (travis-ci/docs-travis-ci-com#910) language that serves the
desired purpose.

.travis.yml

@@ -1,4 +1,4 @@
-language: minimal
+language: generic
 sudo: required
 dist: trusty
 services: