Bug 947596 - Install RBTools from Git; r=mconley

RBTools as released on PyPI has a number of bugs and deficiencies with
the Mercurial implementation. These have been fixed in an unreleased
version of RBTools.

Until RBTools with the desired patches is released to PyPI, we will
install RBTools direct from its source repository.

DONTBUILD (NPOTB)

--HG--
extra : rebase_source : 8fe09b80d8dd5583ba957b4525563cde678b3b3d
extra : amend_source : b41e0d492ff7d1f78bd0462782459c3dc8e95041