travis-ci: switch to Xcode 10.1 macOS image

When building something with GCC installed from Homebrew in the
default macOS (with Xcode 9.4) image on Travis CI, it errors out with
something like this:

  /usr/local/Cellar/gcc/8.1.0/lib/gcc/8/gcc/x86_64-apple-darwin17.5.0/8.1.0/include-fixed/stdio.h:78:10: fatal error: _stdio.h: No such file or directory
   #include <_stdio.h>
            ^~~~~~~~~~

This seems to be a common problem affecting several projects, and the
common solution is to use a Travis CI macOS image with more recent
Xcode version, e.g. 10 or 10.1.

While we don't use such a GCC yet, in the very next patch we will, so
switch our OSX build jobs to use the Xcode 10.1 image.  Compared to
the Xcode 10 image, this has the benefit that it comes with GCC (v8.2)
preinstalled from Homebrew.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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