EE - fix jenkins status check

When using Jenkins CI via the `JenkinsService` class, GitLab queries Jenkins for build status of a specific git commit shasum. This check however is done in an error prone way and fails for Jenkins jobs with multiple modules. Please note the three bubbles at the bottom of this screenshot: http://i.imgur.com/cptQdOG.png

This MR changes the checking code to make use of a HTML parser and selects the element of interest via its xpath. It was manually tested against Jenkins version 1.554 (LTS release as of today) with both maven and freestyle type jobs.