Update mocha to version 5.x (#37)

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
   "devDependencies": {
     "eslint": "^4.0.0",
     "eslint-config-lesshint": "^1.0.3",
-    "mocha": "^4.0.0",
+    "mocha": "^5.0.0",
     "sinon": "^4.0.0",
     "vinyl": "^2.1.0"
   },