Use bcrypt's public cost attr, not internal constant

See:

- https://github.com/codahale/bcrypt-ruby/pull/63
- https://github.com/codahale/bcrypt-ruby/pull/64
- https://github.com/codahale/bcrypt-ruby/pull/65
Merge pull request #12494 from tjschuck/bcrypt_cost_attr

Use bcrypt's public cost attr, not internal constant