.travis.yml: stop requesting libffi & gettext from homebrew

The default package set installed on macOS builders from Travis already
includes libffi and gettext as shown by log messages:

  Skipping install of libffi formula. It is already up-to-date.
  Using libffi
  Skipping install of gettext formula. It is already up-to-date.
  Using gettext
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -40,8 +40,6 @@ addons:
       - gcovr
   homebrew:
     packages:
-      - libffi
-      - gettext
       - glib
       - pixman