Switching to JCenter version of spock-extensions.
Adding fallback to infer version if no ready* tasks run.
Changed nearestversionlocator and inferredversion behavior.

- If multiple tags have they same distance from the HEAD, the locator will
  pick the one with the highest version precedence.
- Version inferrence will no longer fail if distance is 0. In those cases
  the passed scope and stage will be ignored and the nearest version will
  be used. It will additionally marked as un-releasable. The release
  plugin should take this into account and fail the ready task.
Adding check to ready task to ensure version is releasable.