Fix compatibility with Ruby 1.8

While it is recommended to use Ruby 1.9 or 2.0 on workstation, we can
still try to be compatible with 1.8.

Thanks to @edwardchuang for finding this!

lib/knife-solo/cookbook_manager_selector.rb

@@ -38,7 +38,7 @@ def managers
     end
 
     def select_or_disable_by_chef_config!
-      managers.select! do |manager|
+      @managers = managers.select do |manager|
         if (conf = manager.enabled_by_chef_config?)
           Chef::Log.debug "#{manager} selected by configuration"
           return manager