🐛 revert espree update

That was a breaking change that drops Node.js 8 support.

Author: mysticatea

Diff for: package.json

@@ -16,7 +16,7 @@
     "debug": "^4.1.1",
     "eslint-scope": "^5.0.0",
     "eslint-visitor-keys": "^1.1.0",
-    "espree": "^7.0.0",
+    "espree": "^6.2.1",
     "esquery": "^1.0.1",
     "lodash": "^4.17.15"
   },