Bump referenced Browserify version to 5.13

I changed the README to mention the existence of version `0.5` of this gem, together with the fact you can use version `5.x` of browserify with it. 

I haven't tested the actual use of this specific version myself (although I'm starting to work on porting a codebase to it now), so I can also confirm whether or not it actually works in a while.

I've referenced version `5.13` of browserify because it's the last `5.x` release. Is there ever going to be support for `6.x`?