Also test the newest available GCC version.

After commit a72d1200, the Travis-CI
build fails, because we have been using GCC 5 there...  so let's
switch to GCC 6 to test the oldest possible GCC version.

The clang/libc++ version in the default xcode8 image is unable to
compile MPD due to incomplete C++14 support.

Prepare for Mac OS X support by omitting those Ubuntu-specific
commands.

.. and not update-alternatives, which requires "sudo".