fix(package): update through2 to version 3.0.1

Closes #19

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "gulp-util": "^3.0.8",
     "lodash": "^4.17.4",
     "sass-graph": "^2.2.3",
-    "through2": "^2.0.3",
+    "through2": "^3.0.1",
     "vinyl-fs": "^3.0.0"
   },
   "devDependencies": {