Update UglifyJS from 1.x to 2.x

UglifyJS v2 can save more file size as compared with v1.
(cf. http://lisperator.net/blog/should-you-switch-to-uglifyjs2/)

The build result of `build/modernizr-custom.js` (without any options) is here:

| UglifyJS's version        | File size (byte) |
| --------------------------------- |:-----------------------|
| v1.3.3                         | 15,920             |
| v2.4.6                         | 15,807             |

So I updated `package.json` and `lib/gruntifier.js` to use UglifyJS2.