Should use getfixturevalue instead of getfuncargvalue when on pytest 3.0

[mdentremont]

A couple uses of getfuncargvalue should be switched to getfixturevalue when on pytest 3.0 to avoid a deprecation warning:

lib/python3.5/site-packages/pytest_django/fixtures.py
157:        request.getfuncargvalue('transactional_db')
325:        request.getfuncargvalue('transactional_db')

lib/python3.5/site-packages/pytest_django/plugin.py
373:            request.getfuncargvalue('transactional_db')
375:            request.getfuncargvalue('db')
382:        request.getfuncargvalue('django_test_environment')
383:        request.getfuncargvalue('django_db_setup')

[blueyed]

Please submit a PR!

[mdentremont]

@blueyed #393