0.8dev: Fixed regression in r1536964. Refs #706.

`_AssertRaisesContext` can't be imported from `unittest.case` in Python 2.6. Patch by Olemis Lang.


git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1539084 13f79535-47bb-0310-9956-ffa450edef68