Fails if we don't have two challenges

Hi

I'm a very happy user of easywildcard.

For a couple of domains I seen this error

```
Some challenges have failed.
IMPORTANT NOTES:
- The following errors were reported by the server:

Domain: smtp.no.ist.com
Type: dns
Detail: DNS problem: SERVFAIL looking up TXT for
_acme-challenge.smtp.no.ist.com - the domain's nameservers may be
malfunctioning
```
when running `docker run -v /etc/letsencrypt/:/etc/letsencrypt --rm -i -e "EMAIL=email@example.net" -e "DOMAIN=smtp.no.ist.com" easywildcard`

The reason is certbot only gets one challenge, and bind is only startet if there are two challenges, due to https://github.com/Fmstrat/easywildcard/blob/master/hooks/auth-hook.sh's `if [ $NUM -eq 2 ]`

My workaround is to run `docker run -v /etc/letsencrypt/:/etc/letsencrypt -v /tmp/:/tmp --rm -i -e "EMAIL=email@example.net" -e "DOMAIN=smtp.no.ist.com" easywildcard` twice ... this way /tmp/easywildcard/VALIDATION will contain two challanges and bind will start.

Please replace `if [ $NUM -eq 2 ]` with `if [ $NUM -ge 1 ]`

Have a nice day.
Greetings, Christian

Start in hooks/auth-hook.sh and inspect the NUM check that controls when bind starts. Reproduce the single-challenge Docker invocation from the issue, update the condition so one or more challenges start bind, and verify that the validation succeeds without running the container twice.

Written by the indexing model from the issue text.