Update composer.json

Replace IRC reference with Slack
diff --git a/composer.json b/composer.json
index c40d549..30630e2 100644
--- a/composer.json
+++ b/composer.json
@@ -7,7 +7,7 @@
 	"support": {
 		"forum": "http://forum.codeigniter.com/",
 		"wiki": "https://github.com/bcit-ci/CodeIgniter/wiki",
-		"irc": "irc://irc.freenode.net/codeigniter",
+		"slack": "https://codeigniterchat.slack.com",
 		"source": "https://github.com/bcit-ci/CodeIgniter"
 	},
 	"require": {