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;
```

Start by reproducing the webpack failure with the import shown in the issue and inspect color.js, especially its exported isValidHex value. Compare the built module's exports with the source module and verify that color.isValidHex(data.hex) is callable without the TypeError.

Written by the indexing model from the issue text.