line color='none' regression in 1.3

Hi,
The following code works in version 1.2, but leads to an exception in version 1.3:

``` python
>>> from matplotlib import pyplot as plt
>>> plt.plot(range(10), color='none')
>>> plt.show()
```

I think the issue boils down to this commit: https://github.com/matplotlib/matplotlib/commit/edc48f01d27c3f91c2f4460f8fff9eadbd7d2c74

I'm not sure whether this was an intentional change.  Thanks!
