WIP of initializing fallbacks on start

As pointed out in shioyama#481, I18n fallbacks [are lazy-initialized by
default](https://github.com/ruby-i18n/i18n/blob/4709391dceab9096d5988576f93935843023a6ef/lib/i18n/locale/fallbacks.rb#L71).
This can be problematic in production environments, particularly with
forking appservers, since:
1. The first request pays the initialization cost
2. Memory usage is duplicated across appserver processes rather than
   taking advantage of copy-on-write

This is a draft of a way to mitigate these problems. Readability isn't
great since the `i18n` library doesn't seem to have an API to explicitly
initialize fallbacks. Instead we're just looping through each locale and
referencing it so that `i18n` will initialize it.

Considerations:
1. We probably don't want this behavior in development, how should we
   account for that?
2. Ideally this behavior would be configurable, what is the right place
   for that configuration to live? This could potentially address (1).
3. Would this be considered a breaking change to have turned on by
   default?

lib/mobility/plugins/fallbacks.rb

@@ -125,7 +125,12 @@ module Fallbacks
 
       def generate_fallbacks(fallbacks)
         fallbacks_class = I18n.respond_to?(:fallbacks) ? I18nFallbacks : I18n::Locale::Fallbacks
-        fallbacks_class.new(fallbacks)
+        fallbacks_class.new(fallbacks).tap do |fallbacks_instance|
+          fallbacks.each_key do |locale|
+            # Initialize fallbacks
+            fallbacks_instance[locale]
+          end
+        end
       end
 
       class I18nFallbacks < ::I18n::Locale::Fallbacks