Add "engines" to package.json

Enforce Node.js >= 4.5.0 for `Buffer.from()`.

Author: TooTallNate

package.json

@@ -28,5 +28,8 @@
   "devDependencies": {
     "mocha": "3",
     "proxy": "~0.2.3"
+  },
+  "engines": {
+    "node": ">= 4.5.0"
   }
 }