Limit faked GuidePhoto description to 255 chars

Avoids this problem:

  Failure/Error: gp = GuidePhoto.make!(:guide_taxon => gt)
  ActiveRecord::StatementInvalid:
    PG::Error: ERROR:  value too long for type character varying(255)
    : INSERT INTO "guide_photos" ("created_at", "description", "guide_taxon_id", "photo_id", "position", "title", "updated_at") VALUES ($1, $2, $3, $4, $5, $6, $7) RETURNING "id"