GoCD service integration update

[BurkovBA]

Current integration of GoCD with github is obsolete and doesn't work.

I asked GoCD maintainers about it and they pointed me to this github repository and suggested that I fix this: gocd/gocd#4431. I've introduced the appropriate changes to essentially replicate the behaviour of webhook interface.

Unfortunately, I was unable to test this on my local machine. Currently one unit test should surely fail due to invalid webhookSecret value.

Please, take a look.

[ketan]

This will almost entirely break compatibility for existing users of this github service who are using an older version of GoCD. I think we'll need to introduce a change in a manner that works for old and new versions of GoCD, probably with a checkbox that allows for some fallback between different versions of GoCD without breaking compatibility.

[gjtorikian]

Thanks for the pull request! As services have been deprecated for some time, we're only accepting critical security bug fixes. Since GoCD already has a webhook integration, we suggest that users migrate to that path.