Fix return value of Builder.usingServer to a self-reference

It is documented as returning a self-reference (and this is useful
for chaining), but before this commit was instead returning
the string passed to it.

Signed-off-by: Andreas Tolfsen <ato@mozilla.com>

javascript/node/selenium-webdriver/builder.js

@@ -83,7 +83,7 @@ var Builder = function() {
  */
 Builder.prototype.usingServer = function(url) {
   this.url_ = url;
-  return url;
+  return this;
 };