Build without warnings with -pedantic enabled.

[connormanning]

I was trying to bundle amalgamated JsonCpp in a project that is being built with -Werror and -pedantic and had to make this change to avoid a bitfield overflow warning:

  CXX(target) Release/obj.target/project/third/jsoncpp.o
../third/jsoncpp.cpp: In copy constructor ‘Json::Value::Value(const Json::Value&)’:
../third/jsoncpp.cpp:1831:18: error: overflow in implicit constant conversion [-Werror=overflow]
       allocated_ = true;
                  ^
cc1plus: all warnings being treated as errors
make[2]: *** [Release/obj.target/project/third/jsoncpp.o] Error 1

It's kind of a silly change (so feel free to reject) but since it's the only warning emitted I thought it might be worth pulling in.

[cdunn2001]

Interesting. Thanks!