Re-enable direction=up on dropdowns

[pbugnion]

In issue #62 and PR #64 , we decided to disable opening dropdowns with direction="up" until Dash renderer supports React 16.

[tcbegley]

Closing this as it's being tracked by #142.

[tcbegley]

Reopening this, as with the release of Dash 0.41.0, individual issues for outstanding changes for React 16 make more sense than tracking everything through #142

[tcbegley]

Closed by #196