Backport PR
#2686: Fix tox.ini
Prior to this patch, test for Python 3.2 failed.
Also, arguments can be passed to iptest via tox command now.
e.g.: `tox -- IPython.core`
Probably it makes sense to change .travis.yml to something like:
```
language: python
before_install:
- pip install tox
script:
- tox
```
Or you can use `tox -e ...` to run the tests parallel for each Python version.
