This reverts commit fc37928e.

What's been done:
- All the RTM references have not been changed
- All the prerelease refs have been bumped
- All the projects with prerelease refs have been
  changed to use netstandard1.3 instead of dotnet5.4
- Crossgen has been updated to point to the new CLR directory
- The NuSpecs have been updated to use the new refs

There is also now a stand-alone project for referencing
CoreCLR that can be updated independently of corefx. It will
need to be added to the closed solution before the refs in
the existing projects can be removed.

The nuspec currently has a >= dependency on the Microsoft.CodeAnalysis.Compilers
package, which contains the compiler DLLs. It's highly unlikely that csc/vbc.exe
will work with mismatched DLL versions, so this dependency should be fixed, not
minimum.