Docker build no longer works with github url

https://github.com/Kloadut/dokku-pg-plugin/commit/ab9cbab710c2530b76f68c6160965b4fc0651eaa
https://github.com/progrium/dokku/issues/403
https://github.com/dotcloud/docker/issues/3457
Merge pull request #16 from AlJohri/master

Docker build no longer works with github url