src: update NODE_MODULE_VERSION to 56

Major V8 updates are usually API/ABI incompatible with previous
versions. This commit adapts NODE_MODULE_VERSION for V8 5.9.

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

PR-URL: #13263
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

src/node_version.h

@@ -73,6 +73,6 @@
  * an API is broken in the C++ side, including in v8 or
  * other dependencies.
  */
-#define NODE_MODULE_VERSION 55 /* Node.js v8.0.0 */
+#define NODE_MODULE_VERSION 56 /* Node.js v8.0.0 */
 
 #endif  // SRC_NODE_VERSION_H_