chore(package): update sinon to version 8.1.0

Closes #43

Author: greenkeeper[bot]

Diff for: package.json

@@ -56,7 +56,7 @@
     "mocha": "^5.0.0",
     "pouchdb-find": "^6.4.2",
     "pouchdb-memory": "^6.0.0",
-    "sinon": "^6.0.1",
+    "sinon": "^8.1.0",
     "tunnel-agent": "^0.6.0",
     "webpack": "^4.2.0",
     "webpack-command": "^0.4.1"