capistrano-ext dependency

[mikhailov]

Hi

capistrano-ext gem has been included in capistrano and doesn't require to be loaded twice. Once load explicitly it broke some multi-stage settings.

reference: https://github.com/capistrano/capistrano/wiki/2.x-Multistage-Extension

 The multistage extension used to live within a seperate capistrano-ext gem, but is built into capistrano in recent version

Anatoly