Depend on node-gyp explicitly

We're matching the same version that the bundled npm depends on. This
will help us ensure that regardless of how npm organizes the
directories, we will know how to access the node-gyp binary. The risk is
that someone will update npm without updating node-gyp.

Author: Nathan Sobo

package.json

@@ -33,6 +33,7 @@
     "mv": "2.0.0",
     "ncp": "~0.5.1",
     "npm": "3.10.10",
+    "node-gyp": "3.4.0",
     "open": "0.0.4",
     "plist": "git+https://github.com/nathansobo/node-plist.git",
     "q": "~0.9.7",