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

postmodern/chruby,postmodern/chruby