Remove 'if exists' from drop table statement then use `table_exists?`

This pull request addresses `ActiveRecord::StatementInvalid: OCIError: ORA-00933: SQL command not properly ended` errors with Oracle database. Since Oracle database does not support 'drop table if exists' statement.

One of current test case output is here:

```ruby
$ ARCONN=oracle ruby -Itest test/cases/attribute_decorators_test.rb -n test_attributes_can_be_decorated
Using oracle
Run options: -n test_attributes_can_be_decorated --seed 63611

# Running:

E

Finished in 0.138139s, 7.2391 runs/s, 14.4781 assertions/s.

  1) Error:
ActiveRecord::AttributeDecoratorsTest#test_attributes_can_be_decorated:
ActiveRecord::StatementInvalid: OCIError: ORA-00933: SQL command not properly ended: DROP TABLE IF EXISTS attribute_decorators_model
    stmt.c:230:in oci8lib_210.so
    /home/yahonda/.rvm/gems/ruby-2.1.2@rails42/gems/ruby-oci8-2.1.7/lib/oci8/cursor.rb:129:in `exec'
    /home/yahonda/.rvm/gems/ruby-2.1.2@rails42/gems/ruby-oci8-2.1.7/lib/oci8/oci8.rb:278:in `exec_internal'
    /home/yahonda/.rvm/gems/ruby-2.1.2@rails42/gems/ruby-oci8-2.1.7/lib/oci8/oci8.rb:269:in `exec'
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_oci_connection.rb:429:in `exec'
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_oci_connection.rb:88:in `exec'
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_database_statements.rb:10:in `block in execute'
    /home/yahonda/git/rails/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb:458:in `block in log'
    /home/yahonda/git/rails/activesupport/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    /home/yahonda/git/rails/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb:452:in `log'
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:1270:in `log'
    /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_database_statements.rb:10:in `execute'
    test/cases/attribute_decorators_test.rb:31:in `block in <class:AttributeDecoratorsTest>'
    /home/yahonda/git/rails/activesupport/lib/active_support/callbacks.rb:438:in `instance_exec'
    /home/yahonda/git/rails/activesupport/lib/active_support/callbacks.rb:438:in `block in make_lambda'
    /home/yahonda/git/rails/activesupport/lib/active_support/callbacks.rb:257:in `call'
    /home/yahonda/git/rails/activesupport/lib/active_support/callbacks.rb:257:in `block in simple'
    /home/yahonda/git/rails/activesupport/lib/active_support/callbacks.rb:87:in `call'
    /home/yahonda/git/rails/activesupport/lib/active_support/callbacks.rb:87:in `run_callbacks'
    /home/yahonda/git/rails/activesupport/lib/active_support/testing/setup_and_teardown.rb:45:in `after_teardown'
    /home/yahonda/git/rails/activerecord/lib/active_record/fixtures.rb:820:in `after_teardown'

1 runs, 2 assertions, 0 failures, 1 errors, 0 skips
$
```
I understand `if exists` is much simpler and supported by SQLite, PostgreSQL and MySQL, I'd like to execute Rails unit tests with Oracle database to keep support compatibility with Rails. I tested this commit does not cause regressions with sqlite, mysql, mysql2 and postgresql adapters.