ENH: Smaller test dataset (320MB)

We'll see if Travis can actually run this. I have a bad feeling that, with all the additional tests that will be run, we will exceed the ~50-minute limit.
We weren't reading these values in for volume source spaces even though they existed. This makes our support more complete. It actually created a problem when I went to go back and forth between MNE-C created source spaces, and source spaces saved with mne-python.