Upgrade node-pre-gyp to support custom binary hosting mirror

https://github.com/mapbox/node-pre-gyp/pull/170

package.json

@@ -38,7 +38,7 @@
     },
     "dependencies": {
         "nan": "~2.1.0",
-        "node-pre-gyp": "~0.6.14"
+        "node-pre-gyp": "~0.6.18"
     },
     "bundledDependencies": [
         "node-pre-gyp"