docker-ce replaces docker-engine (#642) (#646)
docker-ce replaces docker-engine (#642) (#646)

travis images got updated: docker-engine got replaced by docker-ce
gdbelvin/keytransparency,google/keytransparency,AMarcedone/keytransparency,google/keytransparency,cesarghali/keytransparency,cesarghali/keytransparency,gdbelvin/keytransparency,alsophian/keytransparency,AMarcedone/keytransparency,alsophian/keytransparency