fix cannot modify frozen string error in modern rubys (gregschmit#165)

Author: daflip

lib/helpers/recurring_select_helper.rb

@@ -65,7 +65,7 @@ def ice_cube_rule_to_option(supplied_rule, custom = false)
       ar = [rule.to_s, rule.to_hash.to_json]
 
       if custom
-        ar[0] << "*"
+        ar[0] += "*"
         ar << {"data-custom" => true}
       end