fix deletion order in TestRanking#setup
fix deletion order in TestRanking#setup

If you delete Gems before Value or Ranking,
  Sequel::ForeignKeyConstraintViolation:
  SQLite3::ConstraintException: FOREIGN KEY constraint failed
is raised because Value and Ranking reference Gems.

xmisao/bestgems.org,xmisao/bestgems.org,xmisao/bestgems.org