Added 'template' to call basic_json::any::cast<T>().

I fixed the following compile errors with g++ 4.7.2 (Debian 4.7.2-5) and -std=gnu++0x options:

    jsoncons/src/jsoncons/json2.hpp: In member function ‘const T& jsoncons::basic_json<Char, Storage>::custom_data() const’:
    jsoncons/src/jsoncons/json2.hpp:1379:42: error: expected primary-expression before ‘>’ token
    jsoncons/src/jsoncons/json2.hpp:1379:44: error: expected primary-expression before ‘)’ token
    jsoncons/src/jsoncons/json2.hpp: In member function ‘T& jsoncons::basic_json<Char, Storage>::custom_data()’:
    jsoncons/src/jsoncons/json2.hpp:1395:36: error: expected primary-expression before ‘>’ token
    jsoncons/src/jsoncons/json2.hpp:1395:38: error: expected primary-expression before ‘)’ token