Build: support more Microsoft.FSharp.Targets locations

- Update fsproj files to look for F# 3.1 build targets, with fallback
  to VS targets
- Update sln for VS2013 compatibility

This fixes the `error MSB4057: The target "Build" does not exist in the project.` problem on my machine. It *should* use 3.0 and other targets as per the previous build if it can't find 3.1, but I haven't tested this.