tox.ini: Make more DRY

Using http://tox.readthedocs.org/en/latest/config.html#substitution-for-values-from-other-sections

```
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  py34: commands succeeded
  pypy: commands succeeded
  congratulations :)
```