[tlbuild] Mac OS X build target on Travis CI

[Henri Menke]

Dear list,

I was able to build TeX Live on Travis CI with Mac OS X 10.11 and Xcode
7.3.  I wanted to use Mac OS X 10.10 with Xcode 6.4 but unfortunately
this does not come with XQuartz.  The updated Travis configuration can
be found here:

https://github.com/hmenke/texlive-source/blob/travis/.travis.yml

I would appreciate if you could push this upstream.  Perhaps this can be
used to generate the binaries for future TeX Live releases.

Cheers, Henri