Fixing frustum unit test

Hi,

I saw this unit test broken on dev branch:

![screen shot 2013-08-13 at 12 46 14 am](https://f.cloud.github.com/assets/148319/953122/ade9f184-03eb-11e3-87d0-a54f7d606f6c.png)



This is cause by the fact that  Frustum has a dependency on Sphere on dev branch: https://github.com/mrdoob/three.js/blob/dev/src/math/Frustum.js#L77

Note that it doesn't happen on master branch though: https://github.com/mrdoob/three.js/blob/master/src/math/Frustum.js#L77