add travis-ci status image to README
add travis-ci status image to README

Change-Id: Ibe902f723d1697be575cb0582edaa12f6766bcf9

citrix-openstack-build/cliff,idjaw/cliff,enzochiau/cliff,hguemar/cliff-tablib,varunarya10/cliff,dtroyer/cliff,idjaw/cliff,dtroyer/cliff,openstack/cliff,citrix-openstack-build/cliff,varunarya10/cliff,dreamhost/cliff-tablib,enzochiau/cliff,dhellmann/cliff-tablib,openstack/cliff