Default to Python 3

In support of #8.

Right now I've just installed it and I'm trying to understand the right way to default to python3. There is the option of symlinking both `python` and `pip`.
Looks like statsmodels v0.5.0 supports up through Python 3.2. This container is using Python 3.4.

If I install the current master (slated for `v0.6.0`), it installs just fine.