Rewrite utilities and tasks for Capistrano 3.x

I've patched major part of code to run it successfully with Capistrano 3.

  - Bash scripts inside `CapistranoUnicorn::Utility` have been rewritten using new DSL of Capistrano 3.
  - Tasks have been rewritten for Capistrano 3.
  - Assumed it is used with `capistrano-bundler`.
  - `bundler_gempath` has been removed as it is supported by `capistrano-bundler` gem.
  - Removed unnecessary white spaces.

We should consider removing support for sudo as it is not recommended in Capistrano 3 for better security.