Remove hard dependencies on Clojure, ClojureScript and cljx


I've moved all dependencies and cljx' hook to the :dev profile.

I have to exclude cljx explicitly in my projects, because camel-snake-kebab
currently pulls in cljx (the cljx hook seems to add cljx to :dependencies)

I've removed the dependencies to Clojure and ClojureScript, because
a small utility library like camel-snake-kebab should IMHO not
have a hard dependency on Clojure or ClojureScript at all.

Libraries like Prismatic/schema or ztellman/potemkin do the same.

The tests still pass.