Fix tox python version for Folsom

I just realized that tox is using default Python version for these tests
because it doesn't recognize the names by default.

Change-Id: Idc41847cc5ff881ff0ec78866ac7736f09021727
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Fix tox python version for Folsom"