Fix build on versions between 8.0.0 and 8.6.0

There are conflicting definitions for node::async_id and
node::async_context in Node.js 8.2.

PR-URL: nodejs/node-addon-api#209
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

src/node_internals.h

@@ -70,14 +70,16 @@ class CallbackScope {
 
 namespace node {
 
-#if NODE_MAJOR_VERSION < 8 || NODE_MAJOR_VERSION == 8 && NODE_MINOR_VERSION < 6
+#if NODE_MAJOR_VERSION < 8 || NODE_MAJOR_VERSION == 8 && NODE_MINOR_VERSION < 2
 typedef int async_id;
 
 typedef struct async_context {
   node::async_id async_id;
   node::async_id trigger_async_id;
 } async_context;
+#endif // NODE_MAJOR_VERSION < 8.2
 
+#if NODE_MAJOR_VERSION < 8 || NODE_MAJOR_VERSION == 8 && NODE_MINOR_VERSION < 6
 NODE_EXTERN async_context EmitAsyncInit(v8::Isolate* isolate,
                                         v8::Local<v8::Object> resource,
                                         v8::Local<v8::String> name,