Better fix for #23

There was a typo in the regexp inside ac-nrepl-trim, but even with that
fixed, a nil return value was necessary in order to prevent the reported issue.
Add the class cache function to the end of nrepl-connected-hook

Whatever the cause of issue #24, it seems more correct to add this hook
function *after* `nrepl-enable-on-existing-clojure-buffers`.