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

build/fbcode_builder/travis_docker_build.sh

@@ -11,7 +11,7 @@ travis_cache_dir=${travis_cache_dir:-}
 # The docker build never times out, unless specified
 docker_build_timeout=${docker_build_timeout:-}
 
-cur_dir="$(readlink -f "$(dirname "$0")")"
+cur_dir="$(realpath "$(dirname "$0")")"
 
 if [[ "$travis_cache_dir" == "" ]]; then
   echo "ccache disabled, enable by setting env. var. travis_cache_dir"