Travis CI is reporting: "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."

We should investigate what this means for our builds. The straightforward way would be to switch to the xcode7.3 image, which runs on macOS 10.11. See https://docs.travis-ci.com/user/reference/osx/

This probably means dropping macOS 10.10 support