buildSrc tidy-up. Removed dependency on Gradle.

Also pushed one final class from groovy->java so that the library can be extracted out of this code.
buildSrc tidy-up. Now it is all java code.

Tests are still spock of course!!!
Pushed release-notes script plugin into buildSrc

Needed joint compile java with groovy.
Improved fetching GitHub issues.

We're using a single rest call to acquire the issues.
Removed not-needed file.
Simplified GitHub ticket collection

Started using read-only auth token so that the code runs everywhere and does not require secret env variable. Added test for GitHub api query (not very nice, yet)
Added a TODO and removed jcabi from dependencies