rollback for capistrano tasks

Hello everybody!

@cmer opened the issue #1029 about capistrano rollback a few days ago.
this pull request adds rollback callbacks for the capistrano tasks.

About tests: i didnt found any tests for these tasks. i tried to write some, but the only tool that i found ( https://github.com/fnichol/minitest-capistrano ) doesnt support minitest >= 5.

I'm updating this gem to support minitest >5 , and when i finish, i will write some tests here. ok?