SLC5 builds fail because cannot talk modern https

With the change of EOSWEB from SLC6 to CentOS7, SLC5 docker images cannot do curl http://lhcb-rpm.web.cern.ch/lhcb-rpm/misc/libgit2-0.27.0.tar.gz as it redirects to https:// which only speaks a more recent (and secure) TLS.