Dataset.__repr__ shows array values

Ideally the output would be better aligned between rows and do a better
job of deciding when to show array values rather than just checking if the
array is in memory, but this is good enough for now.

Fixes #206
Removing irrelevant netCDF4 version check
Update travis to test on old pandas
Merge pull request #220 from shoyer/better-formatting

Dataset.__repr__ shows array values