Division of Quantity by a scalar wrong

```
In [1]: from astropy.units import Quantity as Q

In [2]: Q(1, 'm/s') / 2
Out[2]: <Quantity 2.0 s / (m)>
```

This is appears to be doing the inverse of what it should be. FWIW the actual inverse operation is correct:

```
In [3]: 2 / Q(1, 'm/s')
Out[3]: <Quantity 2.0 s / (m)>
```

This just seems to affect division by scalars. Division by other quantities (and curiously, even numpy scalars arrays) is correct:

```
In [12]: Q(1, 'm/s') / np.array(2)
Out[12]: <Quantity 0.5 m / (s)>
```