PERF: only require the rss library if used

Before:

Total allocated: 257909321 bytes (2514134 objects)
Total retained:  39681579 bytes (343387 objects)

allocated memory by gem
-----------------------------------
  42875979  rss

retained memory by gem
-----------------------------------
   2080188  rss

retained objects by gem
-----------------------------------
     13052  rss

After:

Total allocated: 210562047 bytes (2252030 objects)
Total retained:  37433816 bytes (328635 objects)

----

So, 2 less megabytes on boot and 13000 objects stuck in ruby heaps forever.

script/boot_mem.rb

@@ -0,0 +1,21 @@
+# simple script to measure memory at boot
+
+if ENV['RAILS_ENV'] != "production"
+  exec "RAILS_ENV=production ruby #{__FILE__}"
+end
+
+require 'memory_profiler'
+
+MemoryProfiler.report do
+  require File.expand_path("../../config/environment", __FILE__)
+
+  Rails.application.routes.recognize_path('abc') rescue nil
+
+  # load up the yaml for the localization bits, in master process
+  I18n.t(:posts)
+
+  # load up all models and schema
+  (ActiveRecord::Base.connection.tables - %w[schema_migrations versions]).each do |table|
+    table.classify.constantize.first rescue nil
+  end
+end.pretty_print