chore(package): update mocha to version 6.1.2

Closes #66

Author: greenkeeper[bot]

package.json

@@ -26,7 +26,7 @@
     "chai": "4.1.2",
     "ethereumjs-wallet": "0.6.2",
     "ganache-core": "2.2.1",
-    "mocha": "5.1.1",
+    "mocha": "6.1.2",
     "truffle-contract": "3.0.6",
     "web3": "0.20.6"
   },