Remove hardcoded node_modules in gulpfile.
Remove hardcoded node_modules in gulpfile.

1. Use `npx` to execute binaries rather than hardcoded `node_modules/.bin`,
which may not always be the right location.

2. Streamline generateDocumentation by calling it sync.

likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium,progsung/cesium,likangning93/cesium,likangning93/cesium,progsung/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium