src: update NODE_MODULE_VERSION to 68

Major V8 updates are usually API/ABI incompatible with previous
versions. This commit adapts NODE_MODULE_VERSION for V8 7.1.

Refs: https://github.com/nodejs/CTC/blob/master/meetings/2016-09-28.md

src/node_version.h

@@ -109,10 +109,11 @@
  * V8 6.6: 63
  * V8 6.7: 64
  * V8 6.8: 65
+ * V8 7.1: 68
  *
  * More information can be found at https://nodejs.org/en/download/releases/
  */
-#define NODE_MODULE_VERSION 65
+#define NODE_MODULE_VERSION 68
 
 // the NAPI_VERSION provided by this version of the runtime
 #define NAPI_VERSION  3