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)
Merge pull request #574 from opscode/schisamo/ssl-verify-fix

Lock `cacerts` to `2014.08.20`