[fix] Use explicit require of wsEngine (#523)

Because WebPack does not support variables in require statements.

Closes #522

Author: markbjerke

lib/server.js

@@ -104,7 +104,11 @@ Server.prototype.init = function () {
 
   var wsModule;
   try {
-    wsModule = require(this.wsEngine);
+    switch (this.wsEngine) {
+      case 'uws': wsModule = require('uws'); break;
+      case 'ws': wsModule = require('ws'); break;
+      default: throw new Error('unknown wsEngine');
+    }
   } catch (ex) {
     this.wsEngine = 'ws';
     // keep require('ws') as separate expression for packers (browserify, etc)