Upgrade to the latest react-docgen

The latest version of react-docgen includes fixes for support of union types in JSDoc, which would be really great to have supported in react-styleguidist as well.

https://github.com/reactjs/react-docgen/commits/v2.10.0

I discovered this by tracking down the following console error (styleguidist server console - not JavaScript console).

Error when parsing src/components/select/Select.js
TypeError: Cannot read property 'name' of undefined

Here is an example of the offending JSDoc comment: Note the union type of {(string|number)}.

/**
 * Retrieve a value from the given options and valueKey
 * @param {(string|number)} value - the selected value(s)
 * @param {Object} props - the Select component's props (or nextProps)
 */
expandValue(value, props) {
  ...
}

[sapegin]

@nhavar Yep, we’ll see very son.

[sapegin]

Fixed in #189.