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.
Merge branch 'fix/tests' of https://github.com/vzvu3k6k/bestgems.org into vzvu3k6k-fix/tests
Merge branch 'vzvu3k6k-fix/tests'