Update grpc-js to 0.6.18 to fix retry issues in nodejs-pubsub (and elsewhere?)

[feywind]

The recent fixes to grpc-js seem to have fixed some issues, and I'd like to pull those in as a minimum version here to get it into upstream clients.

googleapis/nodejs-pubsub#890

PR coming shortly.

[feywind]

@murgatroid99 Does it look okay to go ahead and update the versions?

[alexander-fenster]

This is out of date and should be closed. We use @grpc/grpc-js v1.0.0+.