README: Fix coveralls badge
README: Fix coveralls badge

And make it point to our master branch...

Signed-off-by: Samuel Ortiz <0ba86cb3f08bbb861958e54bd3438887adb4263c@linux.intel.com>

wcwxyz/virtcontainers,containers/virtcontainers,wcwxyz/virtcontainers,containers/virtcontainers,wcwxyz/virtcontainers