bump rugged to 0.21.0
Merge pull request #968 from ontohub/hotfix-bump_rugged_version

bump rugged to 0.21.0
read correct attribute in migration
Merge pull request #970 from ontohub/hotfix-migration_reads_correct_attribute

read correct attribute in migration
use with_path instead of find_with_path

The latter was replaced by the former.