tox: Invoke the 'pytest' module, not 'py.test'
tox: Invoke the 'pytest' module, not 'py.test'

Follows up e39f9f6 ("tox: change test command to use variable") merged
within pull request #35.

This fixes an error leading to broken Travis builds on Python 3.3:

    python: loader cannot handle py.test

eisensheng/pytest-catchlog