chore(package): update sinon to version 7.3.0

Closes #24

Author: greenkeeper[bot]

package.json

@@ -45,7 +45,7 @@
     "chai": "^3.5.0",
     "express": "^4.14.0",
     "mocha": "^3.3.0",
-    "sinon": "^2.1.0",
+    "sinon": "^7.3.0",
     "sinon-chai": "^2.9.0",
     "supertest": "^2.0.1"
   }