[Fix #206] Fix a false positive for Rails/RakeEnvironment when using Capistrano

[sinsoku]

It excludes the directory where Capistrano tasks are located.
Because these tasks don't need to invoke the :environment task, and the directory structure is the official way.
ref: https://github.com/capistrano/capistrano/tree/v3.12.0#capify-your-project

Fixes #206

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

CI has failed. Can you run rake generate_cops_documentation and apply it?

[sinsoku]

Oops. I updated doc and force-pushed.

[koic]

Thanks!