fix: remove implicit require of uws

So that bundlers like webpack do not try to include it in the build.

As a side-effect, any implementation which matches the API of the ws
module can now be used.

Before that change, you had to explicitly exclude uws:

```
// webpack.config.js
module.exports = {
  // ...
  externals: {
     uws: 'uws'
  }
};
```

Related: #575

Backported from master: 82cdca2

lib/server.js

@@ -102,12 +102,8 @@ Server.prototype.init = function () {
 
   if (this.ws) this.ws.close();
 
-  var wsModule;
-  switch (this.wsEngine) {
-    case 'uws': wsModule = require('uws'); break;
-    case 'ws': wsModule = require('ws'); break;
-    default: throw new Error('unknown wsEngine');
-  }
+  // add explicit require for bundlers like webpack
+  var wsModule = this.wsEngine === 'ws' ? require('ws') : require(this.wsEngine);
   this.ws = new wsModule.Server({
     noServer: true,
     clientTracking: false,