use carmine instead of redis-clojure

Since redis-clojure is no longer being actively developed, carmine has become the de facto redis library for clojure. I've updated the code to use carmine instead. I've also fixed .travis.yml to use lein2 and to start redis-server, so that CI won't fail.

P.S.: Currently there is a test that fails about 1/20 times in token.clj:30. This doesn't seem to be related to my code (it fails on master as well) and I haven't found a fix for it yet. However, this may cause travis to complain about this PR.