README: use single CI badge, remove gitter
README: use single CI badge, remove gitter

Two CI badges are a bit redundant. Since stable is just an older version
of master, and we will now enforce green PRs, its green badge means very
little.

Also, gitter is a 404 so nuke it.

nebolsin/tyk,mvdan/tyk,mvdan/tyk,nebolsin/tyk,mvdan/tyk,nebolsin/tyk,nebolsin/tyk,nebolsin/tyk,lonelycode/tyk,lonelycode/tyk,nebolsin/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk,nebolsin/tyk,mvdan/tyk,nebolsin/tyk