Don't try to decrypt unsupported geli versions

Version 7 is in the wild (at least in FreeBSD 10) and it changed something
about encryption that isn't handled yet:
    http://svnweb.freebsd.org/base?view=revision&revision=238116
presumably earlier versions have other differences I haven't accounted for,
so don't try to read them either.

ungeli.c

@@ -672,6 +672,8 @@ int main(int argc, char **argv) {
     if(!blocksize)
         fatal("Blocksize must not be zero");
 
+    if(md.md_version != 6)
+        fatalf("Unsupported version %d", md.md_version);
 
     if(passphrase_file)
         set_mkey_from_passfile(passphrase_file, &md);