Make test_jenkins_artifacts more forgiving

On some systems, 127.0.0.1 reverse-resolves to an actual hostname, and
not to 'localhost'. Hence, match the output by the IP address instead.
Add Build.get_matrix_runs()

Add a method to get the individual sub-builds for a matrix build.
Merge pull request #162 from kbriggs/topic-matrix-runs

Add method to get sub-builds for a matrix job