Set engine in package.json

[SunBlack]

Currently nowhere is defined which version of nodejs at minimum is required by cesium. This can be defined in package.json as engine .

With node.js 10 you get following following message:
[DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
If we fix this, we don't support node.js 6 anymore, even node.js is still maintained as LTS version. So if we define a minimum version every developer knows if Cesium should still support old version or not.