Move OSX package task into build script

This is to match how the other binaries are built. Also made some
changes to make the Docker containers clean up for when you are running
this locally.

.travis.yml

@@ -27,7 +27,6 @@ before_deploy:
 - git config --local user.name "$USER_NAME"
 - git config --local user.email "$USER_EMAIL"
 - git tag "$VERSION" "$TRAVIS_COMMIT"
-- if [[ “$TRAVIS_OS_NAME” == “osx” ]]; then yarn task package $VERSION; fi
 deploy:
   provider: releases
   file_glob: true