#siblings dies when no :order specified

[garygreyling]

Fix an issue where #siblings crashes with no method error for #quoted_order_column when acts_as_tree was called with no :order option.

[mceachen]

Sweet! I added a failing test: http://travis-ci.org/#!/mceachen/closure_tree/builds/2531476 and pulled your change for this build: http://travis-ci.org/#!/mceachen/closure_tree/builds/2531513

This will be released as v3.6.2 as soon as travis passes.

Thanks!