Ubuntu 14.04 Trusty is LTS until April 2019 and since we are
approaching that, upgrade to Xenial which is LTS until April 2021.

One bonus of this upgrade is that the Travis Xenial virtual images
have a much bigger disk size, thus no need to limit ccache below
the default of 5G anymore.

Also had to remove linux/clang-8 toolchain testing since
llvm-toolchain-xenial is not listed at:
https://github.com/travis-ci/apt-source-safelist/blob/master/ubuntu.json

And fails with:
Disallowing sources: llvm-toolchain-xenial
...
E: Unable to locate package clang-8
E: Unable to locate package llvm-8-dev