chore(package): update sinon to version 7.3.2

Closes #25

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "firebase-functions-test": "0.1.6",
     "chai": "^3.5.0",
     "chai-as-promised": "^6.0.0",
-    "sinon": "^4.1.3"
+    "sinon": "^7.3.2"
   },
   "dependencies": {
     "axios": "^0.18.0",