travis: Fail build if unit tests fail
travis: Fail build if unit tests fail

Before, we used `make coverage` as a proxy for whether any unit tests
failed. However, `make coverage` exits zero even if a unit test failed.
We now run `make check` during the Travis build -- this mirrors what
developers run before submitting PRs, and is simpler than modifying
`make coverage` to exit with the proper exit code.

kelda/kelda,LuiseV/quilt,quilt/quilt,kklin/kelda,ejj/quilt-dev,NetSys/quilt,LuiseV/quilt,kklin/quilt,kklin/quilt,kelda/kelda,NetSys/quilt,quilt/quilt,kklin/quilt,ejj/quilt-dev,kelda/kelda,NetSys/quilt,ejj/quilt-dev,kklin/kelda,quilt/quilt,NetSys/quilt,ejj/quilt-dev,LuiseV/quilt,kklin/kelda,kelda/kelda,LuiseV/quilt,NetSys/quilt,kklin/quilt,quilt/quilt,kklin/kelda,LuiseV/quilt