certbot: Explicitly apt-get update before installing certbot.
certbot: Explicitly apt-get update before installing certbot.

There is no guarantee that the apt data is up-to-date, unless we
explicitly update.

Fixes: zulip/docker-zulip#275

hackerkid/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,zulip/zulip,showell/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,kou/zulip,kou/zulip,zulip/zulip,showell/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,showell/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip