Use `language: generic` in `.travis.yml`

Since the tests don't rely on / fall into any of the language
categories¹ defined by Travis CI, `language: generic`² can be used.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

¹ https://docs.travis-ci.com/user/languages/
² https://github.com/travis-ci/travis-ci/issues/4895#issuecomment-150703192

.travis.yml

@@ -1,4 +1,4 @@
-language: c # defaults to ruby
+language: generic
 addons:
   apt_packages:
   - zsh