float: update vagrant-libvirt cache option
Deprecation came in 0.4.0: https://github.com/vagrant-libvirt/vagrant-libvirt/commit/5471caabe53c2ec19a76121f96ed62a72d701ef0
Deprecation came in 0.4.0: https://github.com/vagrant-libvirt/vagrant-libvirt/commit/5471caabe53c2ec19a76121f96ed62a72d701ef0