Fixes #496. Bind converse to this in callback.

converse.js

@@ -822,7 +822,7 @@
                         return this.ping();
                     }
                     return true;
-                });
+                }.bind(converse));
             }
         };
 

docs/CHANGES.rst

@@ -7,6 +7,7 @@ Changelog
 * #462 Fix MUC rooms with names containing special characters not working [1st8]
 * #468 Fix [object Object] being sometimes shown as status [1st8]
 * #493 Roster wasn't being updated after a Roster push update [teseo, jcbrand]
+* #496 Bugfix. Pings weren't being sent out. [teseo, jcbrand]
 
 0.9.5 (2015-08-24)
 ------------------