Changed download link for pugixml and rapidjson for branch 3.0

pugixml,rapidjson are moved from googlecode with contrail 3.0
Please update the correct link for the same.
Please find the below patch for this.
https://github.com/Juniper/contrail-third-party/pull/40/commits/a8a65626d3d069dea764d12bf6f543c7b83779a0
