travis: remove tip from allowed failures
travis: remove tip from allowed failures

It should be more stable now. I think it's just a matter of time before
it breaks again, but let's see.

davidstack/go-dockerclient,csrwng/go-dockerclient,paulbellamy/go-dockerclient,fsouza/go-dockerclient,tarsisazevedo/go-dockerclient,csrwng/go-dockerclient,davidstack/go-dockerclient,ybogdanov/go-dockerclient,ybogdanov/go-dockerclient,tarsisazevedo/go-dockerclient