chore(package): update sinon to version 8.1.0

Closes #7

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "mocha": "^5.0.0",
     "nyc": "^11.3.0",
     "proxyquire": "^2.0.0",
-    "sinon": "^4.1.2",
+    "sinon": "^8.1.0",
     "sinon-chai": "^3.0.0"
   },
   "scripts": {