README: https clone, not recursive (#3205)
README: https clone, not recursive (#3205)

Using --recursive and SSH no longer necessary when cloning because there
are no more submodules.
Use https, because it's simpler.
netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion