Add travis CI to seaborn

This now runs and claims the build is passing, but that's a bit of a lie at the moment. Two reasons:

- ipynbdoctest exits with 0 even when cells do not replicate, so currently things that should count as failures are masked (I'm not sure what happens when cells error)
- one cell per notebook is currently not replicating, with the following message. I cannot reproduce this on my laptop (all cells reproduce). The failure seems to happen near, but not at, the beginning of each notebook. Maybe the first plotting cell?

    ```
    mismatch output_type:
    stream
      !=  
    display_data
    ```

Nevertheless, merging now as it's running and at least the `nosetests` suite should trigger failures.