Rerun failing tests up to two times on travis-ci
Rerun failing tests up to two times on travis-ci

Travis builds often fail due to errors pulling images needed
for tests. Retry up to two times before failing a test.

MarcoLotz/docker-client,spotify/docker-client,MarcoLotz/docker-client,rgrunber/docker-client,rgrunber/docker-client,spotify/docker-client