src: update NODE_MODULE_VERSION to 60

Major V8 updates are usually API/ABI incompatible with previous
versions. This commit adapts NODE_MODULE_VERSION for V8 6.3.

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

src/node_version.h

@@ -77,6 +77,6 @@
  * an API is broken in the C++ side, including in v8 or
  * other dependencies.
  */
-#define NODE_MODULE_VERSION 58 /* Node.js v8.0.0 */
+#define NODE_MODULE_VERSION 60 /* Node.js v8.0.0 */
 
 #endif  // SRC_NODE_VERSION_H_