chore(package): update sinon to version 8.0.0

Closes #19

Author: greenkeeper[bot]

package.json

@@ -38,6 +38,6 @@
     "express": "~4.16.3",
     "fs-extra": "~6.0.1",
     "mocha": "~5.2.0",
-    "sinon": "~6.0.0"
+    "sinon": "~8.0.0"
   }
 }