Suppress a warning inside boost.

Suppress a warning inside boost.

Specifically, suppress the following warning:

In file included from /u/bangerth/p/deal.II/1/dealii/bundled/boost-1.56.0/include/boost/property_tree/json_parser.hpp:15:0,
                 from /u/bangerth/p/deal.II/1/dealii/source/base/parameter_handler.cc:25:
/u/bangerth/p/deal.II/1/dealii/bundled/boost-1.56.0/include/boost/property_tree/detail/json_parser_write.hpp: In function âstd::basic_string<_CharT> boost::property_tree::json_parser::create_escapes(const std::basic_string<_CharT>&) [with Ch = char]â
:/u/bangerth/p/deal.II/1/dealii/bundled/boost-1.56.0/include/boost/property_tree/detail/json_parser_write.hpp:79:67:   instantiated from âvoid boost::property_tree::json_parser::write_json_helper(std::basic_ostream<typename Ptree::key_type::value_type>&, const Ptree&, int, bool) [with Ptree = boost::property_tree::basic_ptree<std::basic_string<char>, std::basic_string<char> >, typename Ptree::key_type::value_type = char]
â/u/bangerth/p/deal.II/1/dealii/bundled/boost-1.56.0/include/boost/property_tree/detail/json_parser_write.hpp:159:9:   instantiated from âvoid boost::property_tree::json_parser::write_json_internal(std::basic_ostream<typename Ptree::key_type::value_type>&, const Ptree&, const string&, bool) [with Ptree = boost::property_tree::basic_ptree<std::basic_string<char>, std::basic_string<char> >, typename Ptree::key_type::value_type = char, std::string = std::basic_string<char>]
â/u/bangerth/p/deal.II/1/dealii/bundled/boost-1.56.0/include/boost/property_tree/json_parser.hpp:98:9:   instantiated from âvoid boost::property_tree::json_parser::write_json(std::basic_ostream<typename Ptree::key_type::value_type>&, const Ptree&, bool) [with Ptree = boost::property_tree::basic_ptree<std::basic_string<char>, std::basic_string<char> >, typename Ptree::key_type::value_type = char]
â/u/bangerth/p/deal.II/1/dealii/source/base/parameter_handler.cc:1921:32:   instantiated from here
/u/bangerth/p/deal.II/1/dealii/bundled/boost-1.56.0/include/boost/property_tree/detail/json_parser_write.hpp:35:13: warning: comparison is always true due to limited range of data type [-Wtype-limits]