Fixes .sort_by_ancestry with custom ancestry_column

[mitsuru]

Fixes NoMethodError: undefined method ancestry'in.sort_by_ancestry`

[kbrock]

2 parts are not well tested:

• using non id for the primary key. I think rails is pretty good at translating this for us (in read_attribute behind the scenes)
• using a column other than ancestry. These spot tests like you introduced may help a little, but we need to probably bite the bullet and create a suite that runs for a totally different ancestry column.

[kbrock]

Thank you.
I'm going to merge this and then introduce tests that ensure we are testing alternative ancestry columns for the whole suite