snapshotChanges does not inherit type of list/object

[iNViTiON]

Version info

**Angular: 5.2.9 **

Firebase: 4.12.0

AngularFire: 5.0.0-rc.6

The Problem

AngularFireList and AngularFireObject has a generic T that is not inherited to all of it's members, such as snapshotChanges.

The Fix

Need two PR to solve this. One to this project and another to firebase-js-sdk. Submitted here -> firebase/firebase-js-sdk#590

This issue refer and extend to issue1214

#1214

[jamesdaniels]

On master, will cut a @next release with this type additions shortly.