README update - uglify-js instead of uglifyjs

I get an error when trying to install ````uglifyjs````
    
    npm install -g uglifyjs

I get this:

    'uglifyjs' is not in the npm registry

Well, it's ````uglify-js```` and not ````uglifyjs```` ;)
    
    npm install -g uglify-js

Greetings from Germany,
Andre