Bump the clojure-mode dep to 2.0.0.

clojure-mode 2.0.0 was recently released and it's the first version
that target's nrepl.el exclusively. It makes sense that the
upcoming nrepl.el 0.1.6 should require it, given the fact that many
nrepl improvements were incorporated into it.
Merge pull request #212 from bbatsov/clojure-mode-version

Bump the clojure-mode dep to 2.0.0.