Update dev-dependencies

Author: wooorm

package.json

@@ -34,14 +34,14 @@
     "hastscript": "^5.0.0",
     "nyc": "^14.0.0",
     "prettier": "^1.0.0",
-    "rehype-stringify": "^5.0.0",
+    "rehype-stringify": "^6.0.0",
     "remark-cli": "^6.0.0",
     "remark-parse": "^6.0.0",
     "remark-preset-wooorm": "^5.0.0",
     "remark-rehype": "^4.0.0",
     "tape": "^4.0.0",
     "tinyify": "^2.0.0",
-    "unified": "^7.0.0",
+    "unified": "^8.0.0",
     "unist-builder": "^1.0.0",
     "xo": "^0.24.0"
   },