Add immutable task and doctor support

[siutin]

Sorry for the lack of description, Capistrano added these 2 since 3.5.0:

• prevent re-enhancing task after Capistrano is loaded
• add doctor task for troubleshooting purposes

You can verify this here:

• https://github.com/capistrano/capistrano/blob/v3.5.0/lib/capistrano/setup.rb
• https://github.com/capistrano/capistrano/blob/v3.5.0/lib/capistrano/immutable_task.rb

[ayanko]

@siutin CI build failed.

[ayanko]

@siutin Please describe your change in PR. Which problem does it solve?

[siutin]

@ayanko I added the description and fixed the specs.