Revert "Update Roslyn dependency for C# 7.1"
Revert "Update Roslyn dependency for C# 7.1"

This reverts commit e2c6eab55461222fe1f1f6af96c208ace6377624.

aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore