Revert "[Security] Bump nokogiri from 1.8.4 to 1.8.5 (mastodon#8881)"

This reverts commit 0d844c0.

Author: lindwurm

Gemfile.lock

@@ -347,7 +347,7 @@ GEM
       net-ssh (>= 2.6.5)
     net-ssh (4.2.0)
     nio4r (2.3.1)
-    nokogiri (1.8.5)
+    nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
     nokogumbo (1.5.0)
       nokogiri