disk_block_cache: Clarified version checking per akalin-keybase's feedback
disk_block_cache: Clarified version checking per akalin-keybase's feedback

keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client