[chore] Update client location and serve minified file (#2766)

Following socketio/socket.io-client#1025, the
output files are now generated in the dist directory.

lib/index.js

@@ -98,9 +98,9 @@ Server.prototype.serveClient = function(v){
   this._serveClient = v;
 
   if (v && !clientSource) {
-    clientSource = read(require.resolve('socket.io-client/socket.io.js'), 'utf-8');
+    clientSource = read(require.resolve('socket.io-client/dist/socket.io.min.js'), 'utf-8');
     try {
-      clientSourceMap = read(require.resolve('socket.io-client/socket.io.js.map'), 'utf-8');
+      clientSourceMap = read(require.resolve('socket.io-client/dist/socket.io.js.map'), 'utf-8');
     } catch(err) {
       debug('could not load sourcemap file');
     }
@@ -304,6 +304,7 @@ Server.prototype.serve = function(req, res){
   debug('serve client source');
   res.setHeader('Content-Type', 'application/javascript');
   res.setHeader('ETag', expectedEtag);
+  res.setHeader('X-SourceMap', 'socket.io.js.map');
   res.writeHead(200);
   res.end(clientSource);
 };