title: undefined: proto.ProtoPackageIsVersion3
body: <p>I get <code>: ../.../...pb.go:21:11: undefined: proto.ProtoPackageIsVersion3</code>
  I want it to Version 2</p>
  
  <p>I installed <code>protoc</code> from <code>https://github.com/google/protobuf/releases $ protoc --version</code> gives <code>libprotoc 3.6.0</code>. I also installed <code>golang-goprotobuf-dev</code> from <code>apt</code>.</p>
  
  <p>Questions: </p>
  
  <ol>
  <li>protoc (protobuffer compiler compiles always to version Proto3?) </li>
  <li>What is the additional go support required?  </li>
  <li>How can I get a compiler that compiles to Proto2? </li>
  <li>I noticed in one system libproto 3.6.1 compiles to proto2, I don't understand this.</li>
  </ol>