lib: use class syntax
lib: use class syntax

We are targeting ECMAScript 6 now, as a developer tool we don't have to
be that nitty gritty about supporting old versions of node.

This migrates the `Runner` class over to using class syntax.

caspervonb/amok,amokjs/amok,amokjs/amok,caspervonb/amok,caspervonb/amok,amokjs/amok