Lock `cacerts` to `2014.08.20`

The latest version (2014.09.03) causes `berks install` commands to fail with:

```
 /opt/opscode/embedded/lib/ruby/1.9.1/net/http.rb:800:in `connect': SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed (OpenSSL::SSL::SSLError)
```

We only noticed this issue because a new EL6 builder was deployed which hadn't fully warmed it's git cache.

/cc @opscode/release-engineers @opscode/server-team