travis: explicitly require xcode 10.3 image for macOS

travis: explicitly require xcode 10.3 image for macOS

The default macOS image in travis is broken, throwing python
exceptions when trying to install glib. Explicitly ask for the
newer 10.3 image which works correctly.

We now need to also point to the homebrew installed libxml2
rather than the OS distro provided one, since the OS distro
one has a pkg-config file present, but no actual header
files.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>