Revert "Bump the xcode version to work around homebrew update/install shunit2 issue."

* Enable homebrew update to work arount travis-ci / homebrew issue.
  https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296

Author: postmodern

.travis.yml

@@ -5,7 +5,7 @@ matrix:
     - os: linux
       dist: xenial
     - os: osx
-      osx_image: xcode10.12
+      osx_image: xcode9.2
 
 language: c
 
@@ -18,6 +18,7 @@ addons:
   homebrew:
     packages:
       - shunit2
+    update: true
 
 before_script:
   - sudo rm -rf $HOME/.rvm $HOME/.rvmrc