Eliminates global uglify-js dependency in favour of local

[niksumeiko]

After cloning a fresh copy of the project, installing npm dependencies and running npm run build, there's an error thrown.

sh: uglifyjs: command not found
events.js:182
      throw er; // Unhandled 'error' event
      ^

Error: write EPIPE
    at exports._errnoException (util.js:1026:11)
    at WriteWrap.afterWrite [as oncomplete] (net.js:848:14)

Error appears, because project requires uglify-js to be installed globally.

I think, it would be better if we try to avoid relying on global dependencies where possible. This PR does just that — it eliminates global uglify-js dependency in favour of local.