fix(deps): update dependency nuxt to v2.15.8

Author: renovate[bot]

vue-instantsearch/nuxt/package.json

@@ -13,7 +13,7 @@
   "dependencies": {
     "algoliasearch": "4.12.1",
     "cross-env": "^5.2.0",
-    "nuxt": "^2.4.5",
+    "nuxt": "2.15.8",
     "vue-instantsearch": "4.1.1",
     "vue-server-renderer": "2.6.11"
   },