update I18n fallbacks configuration to be compatible with i18n 1.1.0

Gemfile

@@ -45,7 +45,6 @@ gem "dalli"
 gem "listen", ">= 3.0.5", "< 3.2", require: false
 gem "libxml-ruby", platforms: :ruby
 gem "connection_pool", require: false
-gem "i18n", "~> 1.0.1"
 
 # for railties app_generator_test
 gem "bootsnap", ">= 1.1.0", require: false

Gemfile.lock

@@ -274,7 +274,7 @@ GEM
     hiredis (0.6.1-java)
     http_parser.rb (0.6.0)
     httpclient (2.8.3)
-    i18n (1.0.1)
+    i18n (1.1.0)
       concurrent-ruby (~> 1.0)
     image_processing (1.6.0)
       mini_magick (~> 4.0)
@@ -531,7 +531,6 @@ DEPENDENCIES
   ffi (<= 1.9.21)
   google-cloud-storage (~> 1.11)
   hiredis
-  i18n (~> 1.0.1)
   image_processing (~> 1.2)
   json (>= 2.0.0)
   kindlerb (~> 1.2.0)

activesupport/lib/active_support/i18n_railtie.rb

@@ -87,9 +87,21 @@ module I18n
         when Hash, Array
           Array.wrap(fallbacks)
         else # TrueClass
-          []
+          [I18n.default_locale]
         end
 
+      if args.empty? || args.first.is_a?(Hash)
+        ActiveSupport::Deprecation.warn(<<-MSG.squish)
+          Using I18n fallbacks with an empty `defaults` sets the defaults to
+          include the `default_locale`. This behavior will change in Rails 6.1.
+          If you desire the default local to be included in the defaults, please
+          explicitly configure it with `config.i18n.fallbacks.defaults =
+          [I18n.default_locale]` or `config.i18n.fallbacks = [I18n.default_locale,
+          {...}]`
+        MSG
+        args.unshift I18n.default_locale
+      end
+
       I18n.fallbacks = I18n::Locale::Fallbacks.new(*args)
     end