[FIXED JENKINS-24785] - Introduce ROOT_BUILD_CAUSE

For multi-configuration jobs the BUILD_CAUSE passed in the environment
to build scripts is always UPSTREAMTRIGGER. Introduce ROOT_BUILD_CAUSE
that expands UpstreamCause to the root causes.

Update required jenkins version to 1.482 to get
Cause.UpstreamCause.getUpstreamCauses().
Merge pull request #33 from tsondergaard/jenkins-24785-root-cause

[FIXED JENKINS-24785] - Introduce ROOT_BUILD_CAUSE