update dependencies to fix #922

[cruvie]

update dependencies to fix #922

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[cruvie]

fixed in #971

[azabcodes]

I got a new error when using

dependencies:
protobuf: ^3.1.0
grpc: ^4.0.1

The method '$_clearField' isn't defined for the type 'UserRequest'.

and with

dependencies:
protobuf: ^4.0.0
grpc: ^4.0.1

Because grpc >=3.2.1 depends on protobuf >=2.0.0 <4.0.0 and grpc_server depends on protobuf ^4.0.0, grpc >=3.2.1 is forbidden.
So, because grpc_server depends on grpc ^4.0.1, version solving failed.

[cruvie]

@azabcodes grpc/grpc-dart#768