Update tox.ini to usedevelop

The 0.8 hacking tests were failing because it fails to open the
sample files during import. Setting usedevelop=True in the tox.ini
makes the tests run in the right directory.

Change-Id: I7b8f531f669e38e1b009a50d2ed9e9c98cf82e53
Merge "Update tox.ini to usedevelop"