VideoView: Fix a dispatcher leak

Make sure we unregister from the dispatcher when the videoview goes
away.

src/components/views/voip/VideoView.js

@@ -25,8 +25,12 @@ var dis = require('../../../dispatcher');
 module.exports = React.createClass({
     displayName: 'VideoView',
 
-    componentWillMount: function() {
-        dis.register(this.onAction);
+    componentDidMount: function() {
+        this.dispatcherRef = dis.register(this.onAction);
+    },
+
+    componentWillUnmount: function() {
+        dis.unregister(this.dispatcherRef);
     },
 
     getRemoteVideoElement: function() {