Set load paths for dependencies with Makefile

A recent update (036d016 2013-06-02; Rebar dep with no config is given
rebar as manager) introduced :make as a value for a dependency's
manager, but Mix.Deps.load_path has no function clause to handle this.

Add :make to the nil manager clause, as it seems the closest
equivalent.
Merge pull request #1170 from carlosmn/load-path-make

Set load paths for dependencies with Makefile