download minio and mc binaries after they're officially released

[This comment](https://github.com/deis/minio/issues/1#issuecomment-162118057) indicates that the minio team will be doing an official release later this month (12/2015).

Instead of building `mc` and the server from HEAD in the Makefile, the applicable Docker builds should download the binaries from the appropriate location.

This issue relates to https://github.com/deis/builder/issues/11.
