Migrate to a new V8 interceptors Api (nodejs#180)

The new callback should return v8::Intercepted::kYes/kNo to indicate
whether the operation was intercepted. This replaces the old approach
where the callback had to leave the return value unset or set it to
an empty handle to indicate that the the request wasn't intercepted.

See https://crrev.com/c/5465509 and https://crrev.com/c/5465513.
# Conflicts:
#	src/node_contextify.cc

# Conflicts:
#	src/node_contextify.cc
#	src/node_env_var.cc

src/node_env_var.cc

@@ -386,7 +386,6 @@ static Intercepted EnvSetter(Local<Name> property,
   }
 
   env->env_vars()->Set(env->isolate(), key, value_string);
-
   return Intercepted::kYes;
 }