Don't send keepalive if socket has closed

Also shorten the timeout to see if it improves websocket connection
reliability.

js/libtextsecure.js

@@ -15919,15 +15919,17 @@ window.axolotl.sessions = {
         function resetKeepAliveTimer() {
             clearTimeout(keepAliveTimer);
             keepAliveTimer = setTimeout(function() {
+                if (socket.readyState === WebSocket.OPEN) {
                     socket.send(
                         new textsecure.protobuf.WebSocketMessage({
                             type: textsecure.protobuf.WebSocketMessage.Type.REQUEST,
                             request: { verb: 'GET', path: '/v1/keepalive' }
                         }).encode().toArrayBuffer()
                     );
+                }
 
                 resetKeepAliveTimer();
-            }, 60000);
+            }, 55000);
         };
 
         function onclose(e) {

libtextsecure/websocket.js

@@ -39,15 +39,17 @@
         function resetKeepAliveTimer() {
             clearTimeout(keepAliveTimer);
             keepAliveTimer = setTimeout(function() {
+                if (socket.readyState === WebSocket.OPEN) {
                     socket.send(
                         new textsecure.protobuf.WebSocketMessage({
                             type: textsecure.protobuf.WebSocketMessage.Type.REQUEST,
                             request: { verb: 'GET', path: '/v1/keepalive' }
                         }).encode().toArrayBuffer()
                     );
+                }
 
                 resetKeepAliveTimer();
-            }, 60000);
+            }, 55000);
         };
 
         function onclose(e) {