Update the logic for initing a repl on deploy [IMMUTANT-416]

This change makes [:immutant :nrepl-port] override [:ring :nrepl]. The
init now works like this:

1) if no repl config is provided, a repl on port 0 will start in dev mode
2) if [:ring :nrepl] is provided, the repl will only start if [:ring :nrepl :start?]
3) if [:immutant :nrepl-port] is set, it overrides [:ring :nrepl]. If it is `nil`,
   no repl will be started, even in dev mode.