Update Travis CI status badge
Update Travis CI status badge

From travis-ci.org to travis-ci.com.
Travis is in the process of transitioning all repos to travis-ci.com
(used to be for paid accounts only). I just did the transition for
p4lang/p4c in order to fix some build issues (status not reported in a
timely fashion for PRs), which means that the status badge link needs to
be updated.

p4lang/p4c,p4lang/p4c,p4lang/p4c,p4lang/p4c,hanw/p4c,mbudiu-vmw/p4c-clone,hanw/p4c,mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,mbudiu-vmw/p4c-clone,hanw/p4c,hanw/p4c,p4lang/p4c