Firestore test:node:persistence command fails due to #1669 (protobufjs 6.x upgrade).

As of https://github.com/firebase/firebase-js-sdk/pull/1669, "yarn test:node:persistence" fails with:

FirebaseError: [code=unavailable]: Failed to get document because the client is offline.

And in the logs you'll see:

[2019-06-07T17:15:49.191Z] @firebase/firestore: Firestore (6.0.2) [Connection]: GRPC stream error. Code: 13 Message: 13 INTERNAL: Serialization failure

So it seems like the upgrade to protobufjs 6.x broke something. This is non-critical, since node w/ persistence isn't a supported environment externally. But it is very useful for debugging locally and so we should get it passing again.

Once we get it passing, we should probably add it to our "test" command so that it gets picked up by travis as well.