gems: remove cityhash gem.

This gem uses a native extension that requires a C++ compiler to build.
Removing this gem removes the need to have a C++ toolchain to install Danbooru.

app/logical/cache.rb

@@ -37,6 +37,6 @@ class Cache
   end
 
   def self.hash(string)
-    CityHash.hash64(string).to_s(36)
+    Digest::SHA256.base64digest(string)
   end
 end