Added spec element to GhprbTrigger.

By default, the GhprbTrigger XML element generated does not contain a <spec> element. When this element is not present, Jenkins will not use the trigger (tested with Jenkins v1.565, Git client plugin v1.9.1, Git plugin v2.2.1, Github API plugin v1.44, Github Authentication plugin v0.16, Github plugin v1.8, Github Pull Request Builder plugin v1.12).

By default, the spec element seems to get the same value as the cron element (when configured manually).