chore(package): update mocha to version 6.2.1

Closes #9

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
   "devDependencies": {
     "assume": "^2.0.0",
     "istanbul": "0.x.x",
-    "mocha": "4.x.x",
+    "mocha": "^6.2.1",
     "pre-commit": "1.x.x"
   }
 }