Release v1.3.1

Tested versions:

go 1.12.10
protoc 3.9.1

Bug fixes:

• proto/buffer: fix proto.Buffer marshaling.
  • Thanks: https://github.com/apelisse
• plugin/gostring: generate values instead of pointers when a field is repeated and non-nullable.
  • Thanks https://github.com/godfried
• protoc-gen-gogo/generator: Generate json and custom tags for oneof
  • Thanks: https://github.com/krhubert
• plugin/marshalto: Use ProtoSize() in MarshalTo when enabled for oneof fields.
  • Thanks: https://github.com/gaffneyc

Upstream commits:

• 4c88cc3f1a34ffade77b79abc53335d1e511f25b - all: fix reflect.Value.Interface races.
• 6c65a5562fc06764971b7c5d05c76c75e84bdbf7 - jsonpb: fix marshaling of Duration
• b285ee9cfc6c881bb20c0d8dc73370ea9b9ec90f - Log parsing errors using log pkg

Misc:

• add github workflow config
• protoc update - Updated to protoc 3.9.1