Hopefully fix ordering of output on ShiningPanda

Test output on ShiningPanda gets out of order - all the dots appear before any of the 'IPython test group' headings. See for instance [this log](https://jenkins.shiningpanda-ci.com/ipython/job/ipython-win-py27/106/console).

The dots are being printed to stderr, while our own messages are printed to stdout, and must be getting buffered. I think this should fix it.