Add a check to verify if GeoMigrationGenerator works if we're on Rails 5

lib/generators/rails/geo_migration_generator.rb

@@ -2,6 +2,10 @@ require 'rails/generators'
 require 'rails/generators/active_record'
 require 'rails/generators/active_record/migration/migration_generator'
 
+# TODO: this can be removed once we verify that this works as intended if we're
+# on Rails 5 since we use private Rails APIs.
+raise "Vendored ActiveRecord 5 code! Delete #{__FILE__}!" if ActiveRecord::VERSION::MAJOR >= 5
+
 class GeoMigrationGenerator < ActiveRecord::Generators::MigrationGenerator
   source_root File.join(File.dirname(ActiveRecord::Generators::MigrationGenerator.instance_method(:create_migration_file).source_location.first), 'templates')