OL-Cesium 1.31 release

[Guillaume Beraudo]

Hello,

I am pleased to announce version 1.31 of OL-Cesium, based on OpenLayers 4.3.2 and Cesium 1.37.

This version now uses plain npm packages for the OpenLayers and Cesium dependencies.

Closure-util was updated to support goog.modules. The Cesium package comes from the vector tiles branch.

As usual, see CHANGES.md for details.

Thank you to all contributors of OpenLayers, Cesium and OL-Cesium.

Cheers,

Guillaume