security/dehydrated: Update to version 0.6.1-7

Pull in fix for the error "sign_csr:86: challenge_identifiers: assignment
to invalid subscript range".

https://github.com/lukas2511/dehydrated/issues/511

PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226891
Approved by: sascha@root-login.org (maintainer)