Update to Minitest 5
Migrate to newer Minitest assertion methods.

- Use refute_same instead of assert_not_same.
- Use message instead of build_message.
- Use assert instead of assert_block.
- Make assert_nothing_raised call assert instead of raising
  Minitest::Assertion directly (allows the assertion to get counted).
Use a pessimistic version constraint for minitest.
Remove no longer needed rubysl-test-unit dependency.
Use minitest (v5) instead of Test::Unit.
Raise minimum Ruby version to 1.8.7.

minitest v5 is not compatible with Ruby 1.8.6.