register FSharp.Core 4.3.0.0 in gac to fix FsSrGen

Looks like `FsSrGen` has been failing the builds on AppVeyor for the last few days because it can't find FSharp.Core 4.3.0.0. I solved this on @jackfoxy's computer this morning too.

![image](https://cloud.githubusercontent.com/assets/80104/5175216/6819a2ee-73ed-11e4-9cde-3ab0681c3a84.png)