Make import scripts work with frozen strings

script/import_scripts/nabble.rb

@@ -283,7 +283,7 @@ class String
   def indent(count, char = ' ')
     gsub(/([^\n]*)(\n|$)/) do |match|
       last_iteration = ($1 == "" && $2 == "")
-      line = ""
+      line = +""
       line << (char * count) unless last_iteration
       line << $1
       line << $2