chore(package): update testdouble to version 3.1.1

Closes #6

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
     "readme-badges": "^0.3.3",
     "readme-includes": "^0.2.9",
     "stream-buffers": "^3.0.1",
-    "testdouble": "^1.10.2",
+    "testdouble": "^3.1.1",
     "verb": "github:verbose/verb#dev",
     "verb-generate-readme": "^0.3.0"
   },