Update purgecss

[Ffloriel]

Little PR to update to the latest version of purgecss-webpack-plugin compatible with webpack 3.

What would you want me to do with the package-lock.json? I'm usually creating the file .npmrc with package-lock=false but I know people have their own preferences when it comes to handling it.

And thank you for creating the package, it really helps the community use purgecss :)

[sebastiandedeyne]

Perfect as is, thanks!

[sebastiandedeyne]

I assume you meant yarn-lock, that one can stay since it's part of the example.

If you meant package-lock, afaik that doesn't really affect libraries?

[Ffloriel]

I meant package-lock. Yes, it does not affect libraries so some repo add it to the .gitignore or add npmrc file not to generate package-lock. Or some just add the file anyway to the repo.
I just didn't commit the file to this PR.