Fix various compile warnings
Update readme to reflect changes to cider

Calling cljr-update-artifact-cache in the cider-repl-mode-hook no longer
works after the cider repl init code was re-written, this has to done in
the nrepl-conncted-hook instead.  This change was done while preparing for
Cider 0.8, but hopefully it works fine in cider 0.7 too.
Ignore .elc