Fix bug with re-rendering

I'm using `sortable={true}` and saw an error (Line 703, `this._sortable[currentSort.column]` isn't a function) when I first clicked to sort, then caused my parent to re-rendered my Table.

Because the special handling of `if (this.props.sortable === true)` occurs in render *after* `sortByCurrentSort` is triggered by `componentWillReceiveProps`, `this._sortable` was an empty object and it was trying to use the custom-sort code path instead of the default.