gcc-4.6 (Travis CI) does not support

src/lib_json/json_writer.cpp

@@ -961,6 +961,10 @@ int MyStreamWriter::write(Value const& root)
 }
 StreamWriter::Factory::~Factory()
 {}
+StreamWriterBuilder::StreamWriterBuilder()
+  : cs_(StreamWriter::CommentStyle::All)
+  , indentation_("\t")
+{}
 StreamWriter* StreamWriterBuilder::newStreamWriter(std::ostream* stream) const
 {
   std::string colonSymbol = " : ";