Make import scripts work with frozen strings

script/import_scripts/phpbb3/database/database.rb

@@ -26,8 +26,10 @@ module ImportScripts::PhpBB3
         require_relative 'database_3_1'
         Database_3_1.new(@database_client, @database_settings)
       else
-        raise UnsupportedVersionError, "Unsupported version (#{version}) of phpBB detected.\n" \
-          << 'Currently only 3.0.x and 3.1.x are supported by this importer.'
+        raise UnsupportedVersionError, <<~MSG
+          Unsupported version (#{version}) of phpBB detected.
+          Currently only 3.0.x and 3.1.x are supported by this importer.
+        MSG
       end
     end
 

script/import_scripts/phpbb3/importers/poll_importer.rb

@@ -56,7 +56,7 @@ module ImportScripts::PhpBB3
     # @param poll_data [ImportScripts::PhpBB3::PollData]
     def get_poll_text(poll_data)
       title = @text_processor.process_raw_text(poll_data.title)
-      text = "#{title}\n\n"
+      text = +"#{title}\n\n"
 
       arguments = ["results=always"]
       arguments << "close=#{poll_data.close_time.iso8601}" if poll_data.close_time