avoid setting plugin = 'mysql_native_password'

- This is required for MySQL 5.7, but for MariaDB this induces
  some buggy password handling.  dbsake now only sets the plugin
  field when (1) it exists as a field on the mysql.user table
  (not the case for 5.1 and below) and (2) when the major version
  is 5.7.  Otherwise this will be set to the empty string for the
  root@localhost user
- Resolves issue #54