use https to when accessing sonatype maven repository

I'm not sure how closely this is related to the change to `https` for maven repositories in [Leiningen 2.3.4](https://github.com/technomancy/leiningen/commit/e0ad0db0d66dc73e6a04d951791579c5ea4bb75f) but it seems that `https` is available so we should use it I guess.