Upgrade modular-scale to version 2.0.5

modular-scale 1.0.6 really doesn't play nicely with compass 1.0 and sass 3.3. modular-scale 2.0.5 works fine with a few modifications and caveats: 

Caveat: Rounding to whole pixels is no longer supported. We could do this manually with an extra function wrapper  but I have not implemented such a thing in this pull request. 

I have tested with:
   - compass v0.12.6 + sass v3.2.19
   - compass v1.0.0.alpha.19 + sass v3.3.7