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.
  - 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.