Update mod_wsgi tests

Jenkins currently tests PyMongo on the 3.0-dev branch with Python 2.6 and 2.7, with mod_wsgi 2.8 and 3.3.

Update the mod_wsgi tests: replace mod_wsgi 3.3 with 3.4. Add Python 3.4 to the matrix and test it with mod_wsgi 3.4.

The WSGI spec itself is different for Python 3, see PEP 3333. The WSGI scripts we use to test may need to be updated to be compatible with both versions of the WSGI spec.