[node] replace v8::String::Utf8Value with Nan::Utf8String
[node] replace v8::String::Utf8Value with Nan::Utf8String

the used constructor of v8::String::Utf8Value was deprecated
and Nan::Utf8String implements exactly the same interface
as v8::String::Utf8Value. See
https://github.com/nodejs/nan/blob/master/doc/v8_misc.md#api_nan_utf8_string
for more details.

Change-Id: I8ab6f74ae3017a0625f91a196dee3d40ad15acee

bmwcarit/smrf,bmwcarit/smrf,bmwcarit/smrf,bmwcarit/smrf,bmwcarit/smrf,bmwcarit/smrf