Update pug4j to latest version 2.3.1

[chbloemer]

Motivation:

I released a new version of pug4j yesterday and saw vert.x includes an older version. So I updated it for you.

Conformance:

Your commits should be signed and you should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines