Fixed deprecation warning

NanCString is depreacted in nan 1.3
  CXX(target) Release/obj.target/protobuf/src/serialize.o
../src/serialize.cpp:91:43: warning: 'NanCString' is deprecated [-Wdeprecated-declarations]
                                                field->enum_type()->FindValueByName(NanCString(val, &count));
                                                                                    ^
../node_modules/nan/nan.h:2285:33: note: 'NanCString' declared here
NAN_DEPRECATED NAN_INLINE char* NanCString(
                                ^
1 warning generated.