Fix bootstrap: global should be this, not window

In a javascript context, all JS function are not all attached to the window object.
This is the case of 'native' functions. In SlimerJS, the 'require' function is
not attached to the window object. It is a standalone function.

@@ -309,4 +309,4 @@ CasperError.prototype = Object.getPrototypeOf(new Error());
     if (phantom.casperScript && !phantom.injectJs(phantom.casperScript)) {
         return __die('Unable to load script ' + phantom.casperScript + '; check file syntax');
     }
-})(window, phantom);
+})(this, phantom);