Remove make_unique to make the test c++11 compliant

tests/src/unit-custom-base-class.cpp

@@ -173,7 +173,7 @@ TEST_CASE("JSON Node Metadata")
     {
         using json = json_with_metadata<std::unique_ptr<int>>;
         json value;
-        value.metadata() = std::make_unique<int>(42);
+        value.metadata().reset(new int(42));
         auto moved = std::move(value);
 
         CHECK(moved.metadata() != nullptr);