Xenial CI build job notes

["James E. King III" <jk...@apache.org>]

We currently maintain an Ubuntu Bionic (current) and Ubuntu Xenial
(legacy) build job on Travis CI.  It looks like a few languages have
moved forward and dropped some of the older language level support on
Xenial.

I'd like to suggest we start treating the xenial build job as one that
tests the oldest compatible version of a language we can find.  As an
example, dart-1.x is no longer distributed by the dart team.  They now
distribute v2.0 through v2.4.  Further, if a language level is already
being tested on Bionic and we cannot select an older one for Xenial,
we should disable the language on Xenial by removing it from the
Dockerfile.  This will make that build job faster.

Thanks,

Jim