Update for capistrano v3

Nice work here, we've been using your gem, works great. We just upgraded to capistrano version 3, this pull request adds support. Note this will not work with version 2 and is a breaking change, keep that in mind if and when you release. It really was completely rewritten, and it now uses rake for tasks.

There are no tests, but I might try to add some when I get the chance, or if you have an opportunity to do so. Let me know if I should update anything. `Capistrano::CloudFlare.send_request` is probably the only method needing testing.

Thanks!