Remove Ruby 1.9.3 support

[thavelick]

Currently builds are failing on Ruby 1.9.3 because capistrano-cloudflare requires (by default anyway) the latest version of capistrano, however the latest version of Capistrano does not support Ruby 1.9.3 as of this commit: capistrano/capistrano@36b7382

So it follows that this plugin should not either.