chore(package): update sinon to version 7.2.6

Closes #52

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "mocha": "^5.0.0",
     "nyc": "^11.3.0",
     "proxyquire": "^1.8.0",
-    "sinon": "^4.1.2",
+    "sinon": "^7.2.6",
     "sinon-chai": "^2.14.0"
   },
   "scripts": {