Adding back ref that was removed in rc2

[jooj123]

Seems like ref to Select was removed from Async in rc2 - adding it back
I have needed it for my use case for a while.

I know this only caters for the 1 select - can modify to cater for all children if necessary.

I can kind of get around this by doing something like:

<Select.Async
    {...this.props}
 >
    {(asyncProps) => (
        <Select
            {...asyncProps}
            ref={(ref) => (this.selectRef = ref)}
        />
     )}
</Select.Async>

But I feel this seems now is a clunky way to setting a ref for the default Select

[coveralls]

Coverage increased (+0.02%) to 93.033% when pulling d5f2ce2 on jooj123:patch/add-ref into b4a3fd1 on JedWatson:master.

[JedWatson]

This looks good to me @jooj123, thanks!