chore(package): update mock-fs to version 4.12.0

Closes #1155

Author: greenkeeper[bot]

package.json

@@ -58,7 +58,7 @@
     "jshint": "~2.9.1",
     "mocha": "^3.5.3",
     "mocha-lcov-reporter": "^1.2.0",
-    "mock-fs": "4.10.4",
+    "mock-fs": "4.12.0",
     "mockery": "^2.0.0",
     "rimraf": "^2.6.2",
     "serve-static": "^1.12.6",