Update to Jenkins 1.509.1 LTS

This updates the master branch to the latest [Jenkins Long-Term Support release (LTS)](https://wiki.jenkins-ci.org/display/JENKINS/LTS+Release+Line) release 1.509.1; just like the current version this simply resembles the exploded Java Web Archive (.war).

* :information_source: This could normally be merged to the stackato branch directly, but must be cherry picked this time around due to the unrelated preceding commit https://github.com/Stackato-Apps/jenkins/commit/71019271c0dc1ed249e16829c5d2a70d32f9d40e - I'll submit respective pull request right after this.
* :exclamation: This Jenkins Stackato app is currently dysfunctional for production usage due to Jenkins requiring a persistent filesystem service - I'll submit a subsequent pull request to address this.