Backport recent .travis.yml changes (#308)

Backported changes from master:

* b52260d8bf392aa04c48b8c2467a4c034184de86
* 984eef7d6d78e1213d6ea99897343a5059a07c59
* 532519770dea5d353f0b0d718c8881a15c7542df
* 91b0e7d0ca7c59df28f6a6fc1e8eb86a3925b76c
1 file changed