chore(package): update testdouble to version 3.15.0

Closes #13

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
     "mocha": "4.0.1",
     "pre-git": "3.15.3",
     "semantic-release": "7.0.2",
-    "testdouble": "3.2.6"
+    "testdouble": "3.15.0"
   },
   "config": {
     "pre-git": {