.travis.yml: force a brew update for MacOS builds

It looks like the Travis image package databases are out of date
causing the build to error with:

  Error: Your Homebrew is outdated. Please run `brew update`.
  Error: Kernel.exit

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

.travis.yml

@@ -43,6 +43,7 @@ addons:
       - glib
       - pixman
       - gnu-sed
+    update: true
 
 
 # The channel name "irc.oftc.net#qemu" is encrypted against qemu/qemu