Use 2a algo ident until node bcrypt has 2y support

- 2y algo is identical to 2a except for fixing an issue with unicode input
  strings so this should be safe to do.
- A request to add support for the 2y identifier has been opened here:
  https://github.com/ncb000gt/node.bcrypt.js/issues/175 , once this is
  implemented go back to using the origin repo.