Port JSMin fix to handle slash inside square brackets in regex.

The JSMin bundled in Wro4J fails on regexes like `/[/']/`. The second `/` is mistakenly intepreted as the end of the regex.

This bug was fixed on the original C implementation (douglascrockford/JSMin@55809e6d07be4a0d3029d48112d16ef833e55cf7); this is a simple port of the changes.

I ran into the bug while trying to minify [chjj/marked](https://github.com/chjj/marked).