Fix final Node 6 deprecation warnings (#1498)

This was partially addressed by bumping NAN in #1496.

nodejs/node#6216
https://codereview.chromium.org/1839983002/patch/1/10002

Fixes #1484

Author: xzyfer

src/callback_bridge.h

@@ -203,7 +203,7 @@ Nan::MaybeLocal<v8::Function> CallbackBridge<T, L>::get_wrapper_constructor() {
     tpl->InstanceTemplate()->SetInternalFieldCount(1);
 
     Nan::SetPrototypeTemplate(tpl, "success",
-      Nan::GetFunction(Nan::New<v8::FunctionTemplate>(ReturnCallback)).ToLocalChecked()
+      Nan::New<v8::FunctionTemplate>(ReturnCallback)
     );
 
     wrapper_constructor.Reset(Nan::GetFunction(tpl).ToLocalChecked());

src/sass_types/boolean.cpp

@@ -21,7 +21,7 @@ namespace SassTypes
 
       tpl->SetClassName(Nan::New("SassBoolean").ToLocalChecked());
       tpl->InstanceTemplate()->SetInternalFieldCount(1);
-      Nan::SetPrototypeTemplate(tpl, "getValue", Nan::GetFunction(Nan::New<v8::FunctionTemplate>(GetValue)).ToLocalChecked());
+      Nan::SetPrototypeTemplate(tpl, "getValue", Nan::New<v8::FunctionTemplate>(GetValue));
 
       conslocal = Nan::GetFunction(tpl).ToLocalChecked();
       constructor.Reset(conslocal);