Change migration.rb to erb

Avoids rubocop complaining and is more accurate now that it isn't used
as plain old ruby file.

flipper-active_record.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |gem|
   gem.homepage      = 'https://github.com/jnunemaker/flipper'
 
   extra_files = [
-    'lib/generators/flipper/templates/migration.rb',
+    'lib/generators/flipper/templates/migration.erb',
     'lib/flipper/version.rb',
   ]
   gem.files         = `git ls-files`.split("\n").select(&flipper_active_record_files) + extra_files

lib/generators/flipper/active_record_generator.rb

@@ -13,13 +13,18 @@ def self.next_migration_number(dirname)
       end
 
       def self.migration_version
-        if Rails.version.start_with?('5')
-          "[#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}]"
-        end
+        "[#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}]" if rails5?
+      end
+
+      def self.rails5?
+        Rails.version.start_with?('5')
       end
 
       def create_migration_file
-        migration_template 'migration.rb', 'db/migrate/create_flipper_tables.rb', migration_version: migration_version
+        options = {
+          migration_version: migration_version,
+        }
+        migration_template 'migration.erb', 'db/migrate/create_flipper_tables.rb', options
       end
 
       def migration_version