Uncaught TypeError: Cannot read property 'isValidHex' of undefined
When using with webpack since 2.19.3 it breaks when:

```
import color from 'react-color/lib/helpers/color'


... then in code use:
    color.isValidHex(data.hex)
```
causes error.

I guess it is because in color.js there are:
```
exports.isvalidColorString = exports.red = exports.getContrastingColor = exports.isValidHex = exports.toState = exports.simpleCheckForValidColor = undefined;
```