dns/cascade: Update version 4731675c2=>9a1bdbcf4

This is equivalent to 0.1.0-beta6.

This is a breaking release, it is NOT backward compatible with
0.1.0-beta5. State files generated by earlier versions must be removed
before starting the beta6 daemon.

Changelog:
https://github.com/NLnetLabs/cascade/compare/v0.1.0-beta5...v0.1.0-beta6