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.

Gemfile

@@ -25,7 +25,6 @@ gem 'twitter'
 gem 'aws-sdk', '~> 2'
 gem 'responders'
 gem 'dtext_rb', git: "https://github.com/evazion/dtext_rb.git", require: "dtext"
-gem 'cityhash'
 gem 'memoist'
 gem 'daemons'
 gem 'oauth2'

Gemfile.lock

@@ -122,7 +122,6 @@ GEM
       xpath (~> 3.2)
     childprocess (3.0.0)
     chronic (0.10.2)
-    cityhash (0.9.0)
     coderay (1.1.2)
     concurrent-ruby (1.1.5)
     connection_pool (2.2.2)
@@ -426,7 +425,6 @@ DEPENDENCIES
   capistrano-rbenv
   capistrano3-unicorn
   capybara
-  cityhash
   daemons
   delayed_job
   delayed_job_active_record

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