Add fsharp.core build to proto

[KevinRansom]

Proto build used an lkg for it's fsharp.core. This fixes that.
Also updates the lkg to the latest version of the F# compiler.

[KevinRansom]

@dotnet-bot test Ubuntu14.04 Release Build please

[dsyme]

The one big thing that concerns me about this is that we have lost some test coverage for the compiler. If we insert a bug that affects the correct compilation of FSHarp.Core.dll, we won't detect it until much, much later when we adopt that compiler as the LKG. That could be after we've shipped the compiler to the world.

This isn't at all unlikely - FSharp.Core uses a number of features of the compiler that nothing else uses.

Could we still build the final FSharp.Core - the one we ship and test - with the proto compiler?

I understand that this change makes the logic of a from-source build easier. But there would be other ways to achieve that, wouldn't there, such as doing this conditionally?