Fix setup.py cython cache hack for fast all-py versions testing with tox
This looks like it broke during the cython refactor a couple of months ago,
which is a shame, because it allowed to test python-only (no pyx changes)
changes on py26/27/32/33 in about a minute, where travis takes about 15.

There's also the detox project for doing what tox_prll does, but it seems
to have diverged from the latest tox version. perhaps worth it to fix it upstream,
tox_prll is a (useful) hack.

this was  0947ff0 and c8e8779
