Adds a CrumbExclusion for the GitHub WebHook page

The GitHub webhook endpoint should not be protected by the CSRF protection
built into Jenkins. This commit adds a CrumbExclusion filter so that the endpoint
created by c.c.j.GitHubWebHook is not protected using the CSRF crumb protection scheme.

Bumps Jenkins API version minimum amount required for CrumbExclusion.
[FIXED JENKINS-20140] Merge pull request #35