update graceful-fs dep to 4.x

ran into nodejs/node#2026

Author: jbenet

package.json

@@ -21,7 +21,7 @@
   "license": "ISC",
   "dependencies": {
     "convert-source-map": "^1.0.0",
-    "graceful-fs": "^3.0.5",
+    "graceful-fs": "^4.1.2",
     "strip-bom": "^1.0.0",
     "through2": "^0.6.3",
     "vinyl": "^0.4.6"