Change message routing logic to be an implication instead of AND

Previously, a message wouldn't be admitted if it didn't have a prefix,
(logical) AND if the message was equal to the bot's nickname.

Now, a message with a blank prefix passes through, but if the prefix is
the bot's nickname, it does not.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

lib/irc/irc.ex

@@ -77,7 +77,7 @@ defmodule Omnibot.Irc do
     msg = Msg.parse(line)
 
     # Send the message to the router
-    if msg.prefix && (msg.prefix.nick != State.cfg().nick) do
+    if (!msg.prefix) || (msg.prefix.nick != State.cfg().nick) do
       route_msg(self(), msg)
     end
     {:noreply, socket}