CompositeUnit has no 'get_format_name' method

Printing CompositeUnits doesn't seem to work, at least for me:

```python
>>> from astropy.units import (Unit, CompositeUnit)
>>> unit1 = Unit('nrad/s')
>>> unit2 = Unit('Hz(1/2)')
>>> print(CompositeUnit(1, [unit1, unit2], [1, -1]))
ERROR: AttributeError: 'CompositeUnit' object has no attribute 'get_format_name' [astropy.units.format.generic]
...
```

Any help is great.