third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/csharp/src/Google.Protobuf.Conformance/project.json

+{
+  "buildOptions": {
+    "debugType": "portable",
+    "emitEntryPoint": true
+  },
+  "dependencies": {
+    "Google.Protobuf": { "target": "project" }
+  },
+  "frameworks": {
+    "netcoreapp1.0": {
+      "dependencies": {
+        "Microsoft.NETCore.App": {
+          "type": "platform",
+          "version": "1.0.0"
+        }
+      }
+    }
+  }
+}