fix tests to pass on node v11 by updating mock-fs package (see their bug tschaub/mock-fs#256)

Author: David First

package-lock.json

@@ -94,7 +94,7 @@
     "mocha-appveyor-reporter": "^0.4.2",
     "mocha-circleci-reporter": "0.0.3",
     "mocha-junit-reporter": "^1.13.0",
-    "mock-fs": "^4.5.0",
+    "mock-fs": "^4.8.0",
     "prettier-eslint": "^8.8.2",
     "prettier-eslint-cli": "^4.7.1",
     "rewire": "^4.0.1",