travis: Bump Xcode 10 image to 10.2

travis: Bump Xcode 10 image to 10.2

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>