Reverted uniqueness limits for photo/planting association

It seems this interacts badly with the default_scope on postgres (but
    not on sqlite3).  Error message from the logs:

2013-06-01T02:21:05.312099+00:00 app[web.1]: ActiveRecord::StatementInvalid (PG::Error: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
2013-06-01T02:21:05.312099+00:00 app[web.1]: LINE 1: ...photo_id" = 2 AND "plantings"."id" = 181 ORDER BY created_at...
2013-06-01T02:21:05.312099+00:00 app[web.1]:   app/controllers/photos_controller.rb:59:in `create'
2013-06-01T02:21:05.312099+00:00 app[web.1]:                                                              ^
2013-06-01T02:21:05.312099+00:00 app[web.1]: : SELECT  DISTINCT 1 AS one FROM "plantings" INNER JOIN "photos_plantings" ON "plantings"."id" = "photos_plantings"."planting_id" WHERE "photos_plantings"."photo_id" = 2 AND "plantings"."id" = 181 ORDER BY created_at desc LIMIT 1):

For now, we'll just have to rely on the controller (which adds the
association) to keep things unique.