chore: update mock-fs version

The previous version of mock-fs caused an error message when running the
acceptance tests on node 7.x . This version adds support for version 7.x

Close #3977

Author: beeman

package.json

@@ -150,7 +150,7 @@
     "jasmine-spec-reporter": "^2.7.0",
     "minimist": "^1.2.0",
     "mocha": "^2.4.5",
-    "mock-fs": "3.10.0",
+    "mock-fs": "^3.12.1",
     "npm-run": "^4.1.0",
     "npm-run-all": "^3.0.0",
     "object-assign": "^4.0.1",