firestore listeners aren't correctly cleaned up

[barneyb]

Inside FirestoreConnect.componentWillReceiveProps w/ in createFirestoreConnect the wrong set of listeners is cleaned up:

react-redux-firebase/src/firestoreConnect.js

Line 66 in 3e41c91

firestore.unsetListeners(this.prevData)

this.prevData is overridden before the extant listeners are cleaned up, so it ends up leaking. The call to unsetListeners should be made before this.prevData is reassigned.