Don't follow rel links by default, mirroring pip default behavior.

We should probably make this configurable via PantsObtainer in pantsbuild,
but for now I'm disabling it hard, which mirrors pip 1.6.x series semantics.
This is primarily to fix https://issues.apache.org/jira/browse/AURORA-368.
There's really no good reason to pull rel links.  The only exception we have
in practice is elementtree but I would both 1) love to see that dependency
go away and 2) we can always cache it on raw.github.com if we continue to
rely upon its source as a dependency.

Reviewed at https://rbcommons.com/s/twitter/r/293/