Upgrade memdown devDependency from ^4.0.0 to ^5.0.0 (#668)

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "encoding-down": "^6.0.0",
     "hallmark": "^1.0.0",
     "level-community": "^3.0.0",
-    "memdown": "^4.0.0",
+    "memdown": "^5.0.0",
     "nyc": "^14.0.0",
     "pinkie": "^2.0.4",
     "referee": "^1.2.0",