Sporadic test failure on Travis

[cb372]

Example: https://travis-ci.org/github/higherkindness/mu-scala/jobs/661448697#L1335-L1352

java.lang.IllegalStateException: Stream is already completed, no further calls are allowed
	at com.google.common.base.Preconditions.checkState(Preconditions.java:510)
	at io.grpc.stub.ServerCalls$ServerCallStreamObserverImpl.onNext(ServerCalls.java:346)
	at higherkindness.mu.rpc.internal.MonixAdapters$$anon$5$$anon$6.onNext(MonixAdapters.scala:69)
	at monix.reactive.internal.operators.ConcatMapObservable$ConcatMapSubscriber$ChildSubscriber.onNext(ConcatMapObservable.scala:352)
	at monix.reactive.internal.builders.IteratorAsObservable.fastLoop(IteratorAsObservable.scala:147)
	at monix.reactive.internal.builders.IteratorAsObservable.$anonfun$reschedule$1(IteratorAsObservable.scala:99)
	at monix.reactive.internal.builders.IteratorAsObservable.$anonfun$reschedule$1$adapted(IteratorAsObservable.scala:93)
	at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:447)
	at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
[info] - should be able to run client bidirectional streaming services with avro schema
[info] - should be able to run rpc services monadically *** FAILED ***
[info]   io.grpc.StatusRuntimeException: INTERNAL: gRPC frame header malformed: reserved bits not zero
[info]   at io.grpc.Status.asRuntimeException(Status.java:533)

I've seen a few random failures on Travis recently, not sure if they are all the same as this one. I haven't seen it happen locally (yet).