warn_unused_result warnings emitted during installation

[kylef]

 CXX(target) Release/obj.target/protagonist/src/parse_async.o
../src/parse_async.cc:82:21: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
                    resolver->Resolve(Nan::GetCurrentContext(), v8refract);
                    ^~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/parse_async.cc:84:21: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
                    resolver->Reject(Nan::GetCurrentContext(), error);
                    ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/parse_async.cc:98:27: warning: 'Call' is deprecated [-Wdeprecated-declarations]
                callback->Call(2, argv);
                          ^
../../nan/nan.h:1739:3: note: 'Call' has been explicitly marked deprecated here
  NAN_DEPRECATED inline v8::Local<v8::Value>
  ^
../../nan/nan.h:104:40: note: expanded from macro 'NAN_DEPRECATED'
# define NAN_DEPRECATED __attribute__((deprecated))
                                       ^
3 warnings generated.
  CXX(target) Release/obj.target/protagonist/src/parse_sync.o
  CXX(target) Release/obj.target/protagonist/src/protagonist.o
../src/protagonist.cc:10:5: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
    exports->Set(context, Nan::New<String>("parse").ToLocalChecked(), Nan::New<FunctionTemplate>(Parse)->GetFunction(context).ToLocalChecked());
    ^~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/protagonist.cc:13:5: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
    exports->Set(context, Nan::New<String>("parseSync").ToLocalChecked(), Nan::New<FunctionTemplate>(ParseSync)->GetFunction(context).ToLocalChecked());
    ^~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/protagonist.cc:16:5: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
    exports->Set(context, Nan::New<String>("validate").ToLocalChecked(), Nan::New<FunctionTemplate>(Validate)->GetFunction(context).ToLocalChecked());
    ^~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/protagonist.cc:18:5: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
    exports->Set(context, Nan::New<String>("validateSync").ToLocalChecked(), Nan::New<FunctionTemplate>(ValidateSync)->GetFunction(context).ToLocalChecked());
    ^~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
4 warnings generated.
  CXX(target) Release/obj.target/protagonist/src/refractToV8.o
  CXX(target) Release/obj.target/protagonist/src/validate_async.o
../src/validate_async.cc:77:21: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
                    resolver->Resolve(Nan::GetCurrentContext(), v8refract);
                    ^~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/validate_async.cc:79:21: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
                    resolver->Reject(Nan::GetCurrentContext(), error);
                    ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~