Don't include rebar at all

I don't think it's good practice to include a rebar script with the repo (built on a different platform, etc.). Also, it will not build correctly included as a dependency if the user's rebar version is different (and incompatible)

Instead, we should try to make sure the rebar.config file is compatible with the latest rebar at all times.

Also, the priv folder has nothing in it so I removed it.

Finally, I indicated that all the Makefile targets are phony, and use whatever rebar is in the user's path.