Update README.md correcting post-processor name

The REAME.md states that to configure the uglify part, one needs to define a 'js' post-processor with name 'uglifyjs'.
It took me a while to understand that I could not get it working because the post-processor name is 'uglify' (without js) 
as can be seen under lib/config/uglifyjs.js : exports.name = 'uglify';
Merge pull request #440 from s-dubon/patch-1

Update README.md correcting post-processor name.