iOS Safari 10 bug where SockJS couldn't be found

Fixes iOS Safari 10 bug. At the root, this works around a bug where Safari's eval's scope was getting confused.  Something to do with [this issue](https://stackoverflow.com/questions/46036960/evaluated-expression-const-variable-scope-in-safari)

[bug reference](webpack#1090 (comment))

client/socket.js

@@ -5,7 +5,7 @@ const SockJS = require('sockjs-client/dist/sockjs');
 let retries = 0;
 let sock = null;
 
-function socket(url, handlers) {
+const socket = function(url, handlers) {
   sock = new SockJS(url);
 
   sock.onopen = function onopen() {