The macOS system image already provides a version of xz through
homebrew, so we don't need to explicitly install it through
`brew bundle`.
Skipping the `brew update` step reduces the runtime of test jobs
from 6 minutes to 1, so we get test feedback more promptly.
The xz package is really only needed for 'make dist' targets,
so listing it in a Brewfile doesn't help users get started
building the library. Better not to clutter the top-level
directory further.
The DEB package generator isn't available in the cmake version
provided by travis-ci.org, so only skip invoking it there.
Unbreaks cmake build tests after adding macOS coverage.
Test on Linux and macOS with both gcc and clang.
Ubuntu 14.04 (trusty) is now the default travis environment.
Signed-off-by: Ralph Giles <giles@thaumas.net>
