Replace gitter references with discord on documentation (#2453)

CONTRIBUTING.md

@@ -18,7 +18,7 @@ Bower is a large community project with many different developers contributing a
 
 ## Team Meetings
 
-We communicate through a channel on slack: https://gitter.im/bower
+We communicate through a channel on Discord https://discord.gg/0fFM7QF0KpZRh2cY 
 
 If you'd like to attend the meetings, please fill the [support form](http://goo.gl/forms/P1ndzCNoiG), and you'll get an invite.
 
@@ -29,9 +29,9 @@ The issue tracker is the preferred channel for [bug reports](#bugs),
 requests](#pull-requests), but please respect the following restrictions:
 
 * Please **do not** use the issue tracker for personal support requests. Use
-  [Stack Overflow](http://stackoverflow.com/questions/tagged/bower)
-  [Gitter Channel](https://gitter.im/bower/bower)
-  [Mailing List](http://groups.google.com/group/twitter-bower)
+  [Stack Overflow](http://stackoverflow.com/questions/tagged/bower),
+  [Discord Channel](https://discordapp.com/channels/119103197720739842/123728452816732160),
+  [Mailing List](http://groups.google.com/group/twitter-bower),
   (twitter-bower@googlegroups.com), or
   [#bower](http://webchat.freenode.net/?channels=bower) on Freenode.