Stop using ! placeholder in rules.drush.inc

The change record shows that the ! placeholder was removed from Drupal 8 !placeholder removed from t() and format_string()

But rules.drush.inc still uses this placeholder in the arguments to dt(), which is the drush version of t().

This still works, because dt() uses PHP's strtr() function in most cases, and not Drupal's t() function. And because Drush 8 isn't tied to a specific Drupal core version, so it's supposed to work with Drupal 8 as well.

Regardless, going forward, in the Drupal 8 version of rules.drush.inc we should not use the ! placeholder anymore. This will make the rules.drush.inc code compatible with the RulesCommands.php Drush 9 code we are developing - in particular the two versions will share the same set of translatable strings so that translators don't have to translate the same thing twice.

Patch attached.