Explicitly configure the ciphers that we use

The default is `HIGH:!aNULL:!MD5;`. That gives us quite a few potential
ciphers (~64 on current server configuration).

We don’t support SSLv3, so remove the associated ciphers and have 39
fewer ciphers being loaded.

We can also take https://github.com/cloudflare/sslconfig/blob/master/conf 
as a reasonable default for large parts of the internet. This should
still support Windows XP.
Merge pull request #650 from alphagov/feature/fewer-ssl-ciphers

Explicitly configure the ciphers that we use