Update dependency eslint to v5.15.3

Author: renovate-bot

package.json

@@ -87,7 +87,7 @@
     "css-loader": "2.1.1",
     "del": "4.0.0",
     "es6-promise": "4.2.6",
-    "eslint": "5.12.0",
+    "eslint": "5.15.3",
     "eslint-config-prettier": "4.1.0",
     "eslint-config-wpcalypso": "4.0.1",
     "eslint-loader": "2.1.2",