Update dependency eslint-plugin-jsx-a11y to v6

Author: renovate[bot]

package.json

@@ -45,7 +45,7 @@
     "eslint": "4.19.1",
     "eslint-config-airbnb": "18.2.1",
     "eslint-plugin-import": "2.29.1",
-    "eslint-plugin-jsx-a11y": "4.0.0",
+    "eslint-plugin-jsx-a11y": "6.8.0",
     "eslint-plugin-react": "6.10.3",
     "flow-bin": "0.177.0",
     "mocha": "3.5.3",