Add standard web hook headers to Jenkins CI post

Closes #534 (closed)

The Jenkins web hook was not sending the standard web hook headers. Specifically, the gitlab-plugin for Jenkins needs to use the X-GitLab-Event header. This makes our EE service unable to work with the newer GitLab Plugin for Jenkins. See https://github.com/jenkinsci/gitlab-plugin/issues/272

cc/ @rspeicher Do you have some tips on how I can write a spec for this? I stared at it for a while but am not sure how to assert only on the header piece I care about, and not the whole set of post args. Thanks for your help.