Fix ray differential calculation in SpecularTransmit

Deriving in terms of 'w' vs 'wo' result in some sign changes in the values added to 'wi' to get the differential directions.
Additionally, the eta value was the inverted with respect to calculating the differentials (eg. when Dot(wo, n) is positive, meaning the outgoing ray is outside the material and the incoming ray is in the refractive material, we want eta=1/eta). 

This is the code change I mentioned in this post:
https://groups.google.com/forum/#!topic/pbrt/THex27EOrOE