Add a protoc version check into the build helpers.
Add a protoc version check into the build helpers.

This doesn't help end developers using the plugin, but it atleast helps
validate that the generated files checked in are being generated with a current
enough protoc for their JSON support to be correct.

tbkka/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf