Use the index name explicitly provided in a migration when reverting

This pull request supports a feature introduced in rails/rails#8868

It addresses the following error.

```ruby
$ ARCONN=oracle ruby -Itest test/cases/migration_test.rb -n test_drop_index_by_name
Using oracle
Run options: -n test_drop_index_by_name --seed 23114

# Running tests:

E

Finished tests in 0.037093s, 26.9593 tests/s, 0.0000 assertions/s.

  1) Error:
ExplicitlyNamedIndexMigrationTest#test_drop_index_by_name:
ArgumentError: Index name 'index_values_on_value' on table 'values' does not exist
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_schema_statements.rb:168:in `remove_index'
    test/cases/migration_test.rb:474:in `block in test_drop_index_by_name'
    /home/yahonda/git/rails/activesupport/lib/active_support/test_case.rb:64:in `assert_nothing_raised'
    test/cases/migration_test.rb:472:in `test_drop_index_by_name'

1 tests, 0 assertions, 0 failures, 1 errors, 0 skips
$
```