Upgrade plugin to work with Cucumber v4
We want to be able to support the latest versions of Cucumber and Gherkin, so we need to upgrade the dependencies we have on Cucumber and Gherkin.
As these are major changes, there are a couple of things that break and
require changes - fortunately this is just replacing the Formatter
interface with the EventListener
as the Plugin system slightly
changed.
Additionally, major bump the version of this library as we've introduced a breaking change to supported versions, which breaks backwards compatibility.
Closes #8 (closed).