Clean up older versions of config directory

This should work:

```
olddirs=$(ls -1d mmonit-* | grep -E '^mmonit-([0-9.]+)(-(alpine|ubuntu))?$' | sort --version-sort --field-separator=- --key=2,2 -r | tail -n +3)
find ${olddirs} -prune -mtime +30 -ls
```
When it has proved to work for a few versions, add a `-exec rmdir {}` to the end.
