Revert "Revert "Specify language for Travis CI.""

This reverts commit 9e8351bf2e6e7fbfe2a5b69cc8a2b13b1eb75e0c.

I reverted the wrong commit.
Revert "Intentionally failing test to check Travis CI build works."

This reverts commit 9a0c5bcc93c5ab496d528bb4a7231b48ab20f689.

The experiment has worked - now we want the tests to pass again.