Travis enforce the use of the git protocol v2 on their images,
but the 'xcode10' image doesn't handle this correctly, resulting
in the brew packages installation failing:

  $ git config protocol.version
  2
  $ rvm $brew_ruby do brew bundle --verbose --global
  /usr/local/bin/brew tap homebrew/bundle
  ==> Tapping homebrew/bundle
  Cloning into '/usr/local/Homebrew/Library/Taps/homebrew/homebrew-bundle'...
  fatal: unknown value for config 'protocol.version': 2
  Error: Failure while executing; `git clone https://github.com/Homebrew/homebrew-bundle /usr/local/Homebrew/Library/Taps/homebrew/homebrew-bundle --depth=1` exited with 128.
  Error: Failure while executing; `/usr/local/bin/brew tap homebrew/bundle` exited with 1.

The newer 'xcode10.2' beta [*] image doesn't have this limitation.

This image comes with the following brew packages pre-installed,
which extend the current code coverage:
 - libffi
 - libpng
 - libtasn1
 - gnutls
 - jpeg
 - nettle

[*] https://blog.travis-ci.com/2019-02-12-xcode-10-2-beta-2-is-now-available



Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190220193541.24419-1-philmd@redhat.com>
[AJB: re-enabled MacOS build first]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>