Update minimum Node version to 4.5.0

`Buffer.alloc` and `Buffer.from` were only backported then.

Author: charmander

package.json

@@ -43,6 +43,6 @@
   },
   "license": "MIT",
   "engines": {
-    "node": ">= 4.0.0"
+    "node": ">= 4.5.0"
   }
 }