Only require used active_support extensions.

An application I'm dealing with has its own Hash#slice implementation that active_support/all was indirectly blowing away once protobuf was required I think.

Though I'd rather core classes in an application not be indirectly modified via a third party library this patch explicitly requires only used active_support extensions rather than loading (or autoloading) everything in active_support/all.

Cheers,
Shane.