Update eslint to version 6.0.0

Author: depfu[bot]

package.json

@@ -51,7 +51,7 @@
     "babel-preset-env": "^1.7.0",
     "browserify": "^16.2.3",
     "coveralls": "^3.0.2",
-    "eslint": "^5.8.0",
+    "eslint": "^6.0.0",
     "eslint-config-pretty-standard": "^2.0.0",
     "eslint-plugin-import": "^2.14.0",
     "eslint-plugin-prettier": "^3.0.0",