chore(package): update sinon to version 9.0.0

Closes #20

Author: greenkeeper[bot]

package.json

@@ -76,7 +76,7 @@
     "eslint-plugin-standard": "^3.0.1",
     "mocha": "^5.0.0",
     "node-wget-promise": "^0.1.3",
-    "sinon": "^4.5.0",
+    "sinon": "^9.0.0",
     "uuid-validate": "0.0.2",
     "gitbook-cli": "^2.3.2",
     "rimraf": "^2.6.2"