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.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1037 from davidlt/fir-int-build-source-location

cmssw-ib: fetch int-build sources from GitHub