No EF Core 1.1, você precisa usar este código:
public class Startup
{
//...
public void ConfigureServices(IServiceCollection services)
{
//...
services.AddEntityFrameworkNpgsql();
}
}
e
public class DatabaseClass : DbContext
{
//...
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseNpgsql("my connection string");
}
}