chore(package): update testdouble to version 3.9.3

Closes #17

Author: greenkeeper[bot]

package.json

@@ -25,7 +25,7 @@
     "remark-lint": "^6.0.0",
     "remark-preset-lint-recommended": "^3.0.0",
     "tap-spec": "^4.1.1",
-    "testdouble": "3.1.1"
+    "testdouble": "3.9.3"
   },
   "engines": {
     "node": ">=4",