add mocha to dev dependencies

package.json

@@ -47,5 +47,8 @@
     "remix-solidity": "latest",
     "standard": "^10.0.3",
     "web3": "1.0.0-beta.27"
+  },
+  "devDependencies": {
+    "mocha": "^5.1.0"
   }
 }