Add migration_version variable to migration generator for rails 5 compatibility

lib/generators/paperclip/paperclip_generator.rb

@@ -13,7 +13,7 @@ def self.source_root
   end
 
   def generate_migration
-    migration_template "paperclip_migration.rb.erb", "db/migrate/#{migration_file_name}"
+    migration_template "paperclip_migration.rb.erb", "db/migrate/#{migration_file_name}", migration_version: migration_version
   end
 
   def migration_name
@@ -27,4 +27,10 @@ def migration_file_name
   def migration_class_name
     migration_name.camelize
   end
+
+  def migration_version
+    if Rails.version.start_with? '5'
+      "[#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}]"
+    end
+  end
 end

lib/generators/paperclip/templates/paperclip_migration.rb.erb

@@ -1,4 +1,4 @@
-class <%= migration_class_name %> < ActiveRecord::Migration
+class <%= migration_class_name %> < ActiveRecord::Migration<%= migration_version %>
   def self.up
     change_table :<%= table_name %> do |t|
 <% attachment_names.each do |attachment| -%>