README.md: Fix mesos version requirements.
README.md: Fix mesos version requirements.

Signed-off-by: Gurucharan Shetty <a1872e333d0e52644f6125da2276530f7ebe5e77@ovn.org>

shettyg/ovn-mesos,shettyg/ovn-mesos,shettyg/ovn-mesos