Travis CI has new infrastructure using the Google Compute Engine which has both
faster CPUs and more memory, and we've been encouraged to switch as it should
help our build times! The only downside currently, however, is that IPv6 is
disabled, causing a number of standard library tests to fail.

Consequently this commit tweaks our travis config in a few ways:

* ccache is disabled as it's not working on GCE just yet
* Docker is used to run tests inside which reportedly will get IPv6 working
* A system LLVM installation is used instead of building LLVM itself. This is
  primarily done to reduce build times, but we want automation for this sort of
  behavior anyway and we can extend this in the future with building from source
  as well if needed.
* gcc-specific logic is removed as the docker image for Ubuntu gives us a
  recent-enough gcc by default.