Animation loader module is loaded from threejs.org

[binzume]

This can cause unexpected issues with three.js updates. Also, if threejs.org goes down, the demos will stop working.

e.g. https://github.com/binzume/aframe-vrm/issues/3

This should be resolved at build time instead of loading dynamically. however, we don't want to include three.js in the pre-built aframe-vrm.min.js because aframe.js also contains three.js.