cmssw-ib: fetch int-build sources from GitHub

There is an inconsistency. cmssw-validation.spec is fetching the sources
from GitHub, but cmssw-ib.spec still used SVN. Correct the situation and
fetch sources from GitHub.

This also must go into `IB/CMSSW_7_3_X/stable ` because `IB/CMSSW_7_2_X/stable` is not auto-merged into `IB/CMSSW_7_3_X/stable` (yet?).

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>