Drop support for mattermost 3.x

README.md

@@ -72,7 +72,7 @@ Used by at least 3 projects. Feel free to make a PR to add your project to this
 
 ## Requirements
 Accounts to one of the supported bridges
-* [Mattermost](https://github.com/mattermost/mattermost-server/) 3.8.x - 3.10.x, 4.x, 5.x
+* [Mattermost](https://github.com/mattermost/mattermost-server/) 4.x, 5.x
 * [IRC](http://www.mirc.com/servers.html)
 * [XMPP](https://jabber.org)
 * [Gitter](https://gitter.im)