[Certbot-dev] Certbot 0.18.1

[Brad Warren]

We just did a small point release to fix problems caused by
certbot-auto's new installation path. The changelog for this release is:

### Fixed

* If certbot-auto was running as an unprivileged user and it upgraded
from 0.17.0 to 0.18.0, it would crash with a permissions error and would
need to be run again to successfully complete the upgrade. This has been
fixed and certbot-auto should upgrade cleanly to 0.18.1.
* Certbot usually uses "certbot-auto" or "letsencrypt-auto" in error
messages and the User-Agent string instead of "certbot" when you are
using one of these wrapper scripts. Proper detection of this was broken
with Certbot's new installation path in /opt in 0.18.0 but this problem
has been resolved.

More details about these changes can be found on our GitHub repo:
https://github.com/certbot/certbot/milestone/45?closed=1