fixes #170 source index assemblies

The `SourceLink` target was updated to support both frameworks. It will now always run before the `NuGet` target. GitHub now serves code from [raw.githubusercontent.com](https://developer.github.com/changes/2014-04-25-user-content-security/). The old links work, they are just redirected to this new domain.

This source indexing allows the Windows debuggers like Visual Studio to download the code on demand while debugging. FAKE v3 is source indexed and it has been incredibly helpful. Please enable source indexing of FCS assemblies.