The APIs in IConfiguration have changed.

public interface IConfiguration 
{
    string this[string key] { get; set; }
    IConfigurationSection GetSection(string key);
    IEnumerable<IConfigurationSection> GetChildren();
}

There are two specializations of this interface now - IConfigurationRoot and IConfigurationSection.

IConfigurationRoot is used only for the root node and is returned by the ConfigurationBuilder. It has the ability to trigger the reload of the configuration from the configuration sources:

public interface IConfigurationRoot: IConfiguration 
{
    void Reload();
}

Note that the GetSection() and GetChildren() methods now return a new IConfigurationSection interface that represent child configuration "sections". This is the definition of the interface:

public interface IConfigurationSection: IConfiguration 
{
    string Key { get; }
    string Value { get; set; }
}

See aspnet/Configuration#277 for discussion