Firefox CirclePicker white pixel bug
Status:
![image](https://user-images.githubusercontent.com/6824340/75308710-b0e45180-5881-11ea-84ae-1342ef083224.png)

Cause:
https://github.com/casesandberg/react-color/blob/6a7f338ad46fa2ac9b0635091d2a7f798f5513d5/src/components/circle/CircleSwatch.js#L20

Solution:
assign background same color with box shadow. Tested in chrome and firefox.
```
background: color,
```