According to:

  https://docs.travis-ci.com/user/reference/osx/#macos-version



we have 10.3 available so lets use it. I don't know what Apple's
deprecation policy is for Xcode because it requires an AppleID to find
out.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>