[thor-16-17-support] Support Thor 0.16-0.17

Currently in the gemspec anything > Thor 0.16.0 is allowed, however in Thor 0.18.0 current_task was renamed to current_command and omnibus is making use of current_command only so it was failing with a no method .name for nil class. This change makes it truly compatible with Thor 0.16-0.17, etc.