Bump NODE_MODULE_VERSION to 54

This matches the number in nodejs/node#10992.

Author: zcbenz

src/node_version.h

@@ -52,6 +52,6 @@
  * an API is broken in the C++ side, including in v8 or
  * other dependencies.
  */
-#define NODE_MODULE_VERSION 51 /* Node.js v7.0.0 */
+#define NODE_MODULE_VERSION 54 /* Node.js v7.0.0 */
 
 #endif  // SRC_NODE_VERSION_H_