repair Clang 5, 6 Travis builds

A couple of weeks ago, Travis CI upgraded their default Linux environment from
Ubuntu Trusty 14.04 to Ubuntu Xenial 16.04 [0]. Clang 5 and 6 builds started
failing because the llvm-toolchain repositories we're using don't support
Xenial. I suspect other Clang builds that are passing (e.g. Clang 4) are only
doing so coincidentally. I didn't investigate the failures initially, assuming
they were yet another transient Travis failure, and when I did it was
surprisingly hard to find the answer [1].

The fix in this commit is to pin these builds to Trusty. In future we should
also (1) pin the other Linux builds and (2) extend testing to Xenial.

  [0]: https://changelog.travis-ci.com/xenial-as-the-default-build-environment-99476
  [1]: https://travis-ci.community/t/cannot-apt-get-install-clang-5-0/3250/3