ROOT: Add HEAD support as a stopgap for 10.9 users

This commit adds a HEAD URL/branch for additional patches to the current
ROOT release.  The HEAD branch points to the *official* ROOT GitHub
mirror instead of the CERN git server simply because the CERN git server
is primarily for internal use.  By specifying --HEAD, users should be
able to build ROOT on OS X 10.9.  At the moment this is the only way to
build ROOT on 10.9 because the official release is heavily tied to
libstdc++'s internals.