protobuf3_1: init at 3.1.0

pkgs/development/libraries/protobuf/3.1.nix

···
+{ callPackage, ... }:
+
+callPackage ./generic-v3.nix {
+  version = "3.1.0";
+  sha256 = "0qlvpsmqgh9nw0k4zrxlxf75pafi3p0ahz99v6761b903y8qyv4i";
+}

pkgs/top-level/all-packages.nix

···
   protobuf3_0 = lowPrio (callPackage ../development/libraries/protobuf/3.0.nix { });
   # 3.0.0-beta-2 is only introduced for tensorflow. remove this version when tensorflow is moved to 3.0.
   protobuf3_0_0b2 = lowPrio (callPackage ../development/libraries/protobuf/3.0.0-beta-2.nix { });
+  protobuf3_1 = callPackage ../development/libraries/protobuf/3.1.nix { };
   protobuf2_6 = callPackage ../development/libraries/protobuf/2.6.nix { };
   protobuf2_5 = callPackage ../development/libraries/protobuf/2.5.nix { };