Specify Draft3Validator rather than rely on default

jsonschema 1.1.0 switched to using draft 4 of the JSON Schema specification by
default. We're still using draft 3. This change makes that explicit, using
syntax that is compatible with versions 0.7.0 all the way to the current tip
(1.2.0 beta).

Change-Id: I80ae64a0269303aa1852516dbea563a8f0df752a