chore(package): update sinon to version 8.0.0

Closes #31

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
   "devDependencies": {
     "mocha": "^5.0.0",
     "proxyquire": "^1.7.11",
-    "sinon": "^5.0.0",
+    "sinon": "^8.0.0",
     "standard": "^10.0.1",
     "semantic-release": "^7.0.1"
   },