Danger dependencies resolver

[f-meloni]

Given Marathon is now deprecated JohnSundell/Marathon#208
I've though was the case for Danger to bring the part it needed of Marathon in the project, and keep maintaining it.
I've taken this opportunity also to make the code more modular, testable and optimised.
This also removes a lot of dependencies, that means the build time of Danger will be a lot faster.
But is still 100% compatible with the Marathon cache of the previous version.

To compare the two version I've used the Moya repo

This is when there is no cache

This is when there is a cache

And those two are the same tests with the old code

After this PR is merged I will anyway make a new major release

[sunshinejr]

this is huge!! didn't have time to go through it all, but it's a massive amount of work (oh and also congratz on the perf!) @f-meloni 👏

[orta]

Very cool