updated README file to solve 'useref.restore()' issue

When the example provided in the README file is executed, [this](http://pastebin.com/95BqWPKM) error occurs. Assigning 'useref.assets()' to a local variable solves the problem. My package versions are as follows;
 - gulp: 3.8.8
 - gulp-use-ref: 1.0.2