Restored protobuf.js Message type as generic

[FedeBev]

Fix the error

error TS2314: Generic type 'Message<T>' requires 1 type argument(s).

Message type is defined in the protobufjs package as below

export class Message<T extends object> {
    ...
}

so it should be a generic

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[FedeBev]

All done, I signed the CLA.

[nicolasnoble]

Please comment on #425 as to why #176 that got solved using #393 was inappropriate.

[FedeBev]

That's wrong because this package use protobufjs 5, in this version Message is not a generic. So #393 it's right and we have to import types for protobufjs 5 in our projects.

Maybe we should add a note in README.md.