Make `travis_docker_build.sh` macos-compatible
Make `travis_docker_build.sh` macos-compatible

Summary:
BSD readlink doesn't have -f.

I'm not using TravisCI, however docker is still convenient for reproducing builds locally.

Reviewed By: yns88

Differential Revision: D29523333

fbshipit-source-id: e01169f3eabca7b8baec95bc70fe119cad201b35

facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman