Add *.gemfile to Bundler cop target

[sue445]

Creating files with the .gemfile extension is well done.

e.g)

• https://github.com/kaminari/kaminari/tree/master/gemfiles
• https://github.com/activeadmin/activeadmin/tree/master/gemfiles
• https://github.com/charliesome/better_errors/tree/master/gemfiles
• https://github.com/thoughtbot/factory_bot_rails/tree/master/gemfiles

So I thought good that rubocop will check .gemfile.

---

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 rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.