src: update NODE_MODULE_VERSION to 51

When V8 was updated on master to 5.4 there were ABI breaking changes.
In the past we have not landed these types of changes before a release,
and as such have only bumped the NODE_MODULE_VERSION number in the
release commit.

Since we are going to be keeping the V8 5.4 beta on master and in the
v7 betas I think it makes sense for us to bump the module number prior
to a release commit being made. It is possible that this commit should
be reverted prior to v7.0.0 being cut. Alternatively we may want to
modify our release process for V8 to include a NODE_MODULE_VERSION
bump before landing on master when applicable.

NODE_MODULE_VERSION is being bumped to 51 instead of 49 to avoid
conflicts with NODE_MODULE_VERSIONs being used in electron.

Ref: https://github.com/electron/electron/issues/5851#issuecomment-246920775
Ref: https://github.com/nodejs/node/pull/8317

PR-URL: https://github.com/nodejs/node/pull/8808
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/node_version.h

@@ -49,6 +49,6 @@
  * an API is broken in the C++ side, including in v8 or
  * other dependencies.
  */
-#define NODE_MODULE_VERSION 48 /* Node.js v6.0.0 */
+#define NODE_MODULE_VERSION 51 /* Node.js v7.0.0 */
 
 #endif  // SRC_NODE_VERSION_H_