Rename nrepl-mode to nrepl-repl-mode to match upstream nrepl.el

nrepl.el summarily renamed nrepl-mode to nrepl-repl-mode, so this should
fix compatibility problems referencing nrepl-mode var's.

This appears to be the cause of the following bug report: https://groups.google.com/forum/#!topic/nrepl-el/smnLFjxhBNs