Use a more recent StartSSL guide instead

This replaces the link to the [duraconf StartSSL guide](https://github.com/ioerror/duraconf/blob/master/startssl/README.markdown) with a link to [my StartSSL guide](https://konklone.com/post/switch-to-https-now-for-free). 

A self-serving pull request! But the duraconf guide hasn't been updated in 3 years, mine was made in September and I update it whenever I find relevant things. I think it's pretty good, and it's been well-used and gets steady traffic.