Travis deprecated xcode 6.4 (used for wheel files)

I noticed the following lines in the travis logs (used from: https://github.com/apache/beam-wheels)

Running builds with Xcode 6.4 in Travis CI is deprecated and will be removed in January 2019.
If Xcode 6.4 is critical to your builds, please contact our support team at support@travis-ci.com to discuss options.
Services are not supported on osx

We need to change the version we are using to build wheel files.
Lots of newer versions are already supported: https://docs.travis-ci.com/user/reference/osx/#macos-version

robertwb[boyuanz is there a reason for us to use xcode 6.4 here?

(Not release blocking because builds are still working even though there is a warning.)