chore(package): update eslint to version 6.7.1

Closes #57

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "through2": "^3.0.0"
   },
   "devDependencies": {
-    "eslint": "^5.0.0",
+    "eslint": "^6.7.1",
     "eslint-config-lesshint": "^1.0.3",
     "mocha": "^6.0.0",
     "sinon": "^7.2.2",