Bump grpcio to 1.16.1

[c24t]

This fixes a version incompatibility problem with the grpc hello world example.

To reproduce on master, run the server:

GOOGLE_APPLICATION_CREDENTIALS=/your/creds.json GOOGLE_CLOUD_PROJECT=your-gcloud-project python3 \
 ./examples/trace/grpc/hello_world_server.py

...and watch it crash when you run the client:

GOOGLE_APPLICATION_CREDENTIALS=/your/creds.json GOOGLE_CLOUD_PROJECT=your-gcloud-project python3 \
 ./examples/trace/grpc/hello_world_client.py

ERROR:root:Exception calling application: 'grpc._cython.cygrpc.Event' object has no attribute 'call_details'
Traceback (most recent call last):
  File "/usr/local/brew/lib/python3.7/site-packages/grpc/_server.py", line 377, in _call_behavior
    return behavior(argument, context), True
  File "/usr/local/brew/lib/python3.7/site-packages/opencensus/trace/ext/grpc/server_interceptor.py", line 46, in new_behavior
    span = self._start_server_span(servicer_context)
  File "/usr/local/brew/lib/python3.7/site-packages/opencensus/trace/ext/grpc/server_interceptor.py", line 111, in _start_server_span
    name=_get_span_name(servicer_context)
  File "/usr/local/brew/lib/python3.7/site-packages/opencensus/trace/ext/grpc/server_interceptor.py", line 167, in _get_span_name
    method_name = servicer_context._rpc_event.call_details.method[1:]
AttributeError: 'grpc._cython.cygrpc.Event' object has no attribute 'call_details'

We should consider running the examples as part of CI or pinning the test dependencies to prevent this happening again.