Update tooltips rendered in portal. Fix #793.

[parkerziegler]

This PR should fix issue #793 Tooltips in VoronoiContainer stay showing after leaving chart. Tooltips rendered in a portal were not being removed because render was never being called on the <Portal /> component after the portal was "deregistered." Calling this.forceUpdate after unregistering the portal forces a re-render, which returns an empty object for the children that should be rendered in the portal and removes the tooltip. For example:

Notice that the tooltips are being rendered in the <Portal />, but they are properly unmounting when the user leaves the bounding area.

Local tests for this code were performed using the lank workflow, i.e. changes were made to victory-core and tested using victory-voronoi-container-demo.js from a sibling local version of victory-chart. To get the tooltips to render in the portal, comment out this line from victory-voronoi-container.js in your local install of victory-chart.

[david-davidson]

Good find!