replacing std::to_string by boost::lexical_cast<std::string>.

There is a compiler bug that is not fixed by update in Zeuthen, yet.
http://stackoverflow.com/a/12975602/2066546

References:
* http://stackoverflow.com/a/1123216/2066546
* http://www.boost.org/doc/libs/1_39_0/libs/conversion/lexical_cast.htm