Update Homebrew to avoid failed builds

dspinellis · dspinellis · commit ee477186d589 · 2020-03-22T20:25:20.000+02:00
See https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296/10
diff --git a/.travis.yml b/.travis.yml
@@ -7,6 +7,7 @@ addons:
     packages:
       - shellcheck
       - jq
+    update: true
 
 language: bash
 
