invalid cert chain for oligarchy.co.uk

Hi all,

At some point the certificate chain for oligarchy.org became invalid and
it’s no longer possible to securely download files:

```
$ curl -IL https://oligarchy.co.uk/xapian/1.4.3/xapian-core-1.4.3.tar.xz

curl: (60) SSL certificate problem: Invalid certificate chain
More details here: https://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.
```

Since http:// redirects to https://, apparently we can’t connect at all: https://www.ssllabs.com/ssltest/analyze.html?d=oligarchy.org