ci: travis: ensure we update brew as well to avoid a problem on Travis

We normally did not use update as it could take up a few minutes for
every macos build and the packages have been recent enough. Since a few
hours we see build problems now on Travis:
Error: Unknown command: bundle

A way to get them fixed (until Travis updates their images for macos) is
to use the update command.
https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296/9

Also moving to a newer osx image here to avoid falling further behind on
the dependencies.

.travis.yml

@@ -7,7 +7,7 @@ cache:
     - $HOME/.ccache
 dist: bionic
 
-osx_image: xcode10.2
+osx_image: xcode11.3
 
 addons:
   homebrew:
@@ -36,6 +36,7 @@ addons:
       - ninja
       - python3
       - libffi
+    update: true
 
 env:
   global: