chore(package): update sinon to version 7.5.0

Closes #84

Author: greenkeeper[bot]

package.json

@@ -34,7 +34,7 @@
     "mocha": "~3.4.1",
     "nock": "~9.0.6",
     "power-assert": "~1.4.2",
-    "sinon": "~2.3.0"
+    "sinon": "~7.5.0"
   },
   "directories": {
     "test": "test/"