Add post_install_message about change of fallbacks

[kyamaguchi]

We experienced problem with our projects by the change of fallbacks behavior.
We wanted to know the breaking change.

I want any users who will update i18n to know the change to avoid troubles.
I don't mind the detail(content) of post_install_message.
I borrowed an idea from the other popular gem haml.
https://github.com/haml/haml/blob/f699ca90cf5ce09f71035aebe8c3b6d67401ebfa/haml.gemspec#L37

Main issue:
#415

Changes in Rails 6:
rails/rails#34383
rails/rails#33574

---

Not all users are affected by the fallbacks issue. (Affected users are much less.)
So post_install_message could be too much because it will be displayed to all users.

The better alternative may be displaying warning message.

lib/i18n/locale/fallbacks.rb

@@ -56,7 +56,7 @@ module I18n
       def initialize(*mappings)
         @map = {}
         map(mappings.pop) if mappings.last.is_a?(Hash)
-        self.defaults = mappings.empty? ? [] : mappings
+        self.defaults = mappings.empty? ? empty_with_warning : mappings
       end

       def defaults=(defaults)
@@ -91,6 +91,13 @@ module I18n
         result.push(*defaults) if include_defaults
         result.uniq.compact
       end
+
+      private
+
+      def empty_with_warning
+        puts "Warning message"
+        []
+      end
     end

Rails introduced message but it doesn't help the majority of current Rails users who use Rails (< 6.0).
https://github.com/rails/rails/pull/33574/files

[radar]

I think this should point to the exact release that broke it.

[radar]

Thank you @kyamaguchi. I agree that we should add a post install message. I've got one small suggestion and then I think this PR is good to go.

[kyamaguchi]

@radar

I've got one small suggestion

Fixed

[radar]

Thank you! I’ll merge this when I am back at work on Monday.