chore(package): update mocha to version 6.2.1

Closes #31

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "chai": "^3.5.0",
     "errno-codes": "^1.0.2",
     "istanbul": "^0.4.5",
-    "mocha": "^3.0.2",
+    "mocha": "^6.2.1",
     "proxyquire": "^1.7.10",
     "rimraf": "^2.5.4",
     "sinon": "^1.17.6",