re-enable tests that were failing on jenkins

The failures were due to a bug in java.time
that was fixed in 8u20. See #9 for details.

This change also adds some assertions on the
status code for the response in the graph
api test so we don't get the confusing exception
about png headers.
Merge pull request #25 from brharrington/jenkins-png-issue

re-enable tests that were failing on jenkins