Upgrade Caddy and make future upgrades easier

Roles/www_alum/tasks/main.yml specifies Caddy 0.8.2.

Alum.recurse.com is running Caddy 0.8.3 (presumably it was installed manually):

```
root@ip-172-31-24-170:/etc/service/caddy# ./caddy --version
Caddy 0.8.3
```

The newest version of Caddy is 0.9.5. A number of Caddy versions have made changes and improvements to how Caddy handles TLS. 0.9.0 has some breaking changes, and 0.9.5 has some critical fixes.

We should upgrade to Caddy 0.9.5 in a way that makes it easy to upgrade Caddy versions in the future, and doesn't end up with a different version installed than is specified in the Ansible playbook.