Fix rake new_cop problem that doesn't add require line

[koic]

This PR fixes the following problem of rake new_cop.

For example use rake new_cop to add a new Style/TheEndOfStyle cop.

Expected Behaviour

The following two files are generated.

Files created:
  - lib/rubocop/cop/style/the_end_of_style.rb
  - spec/rubocop/cop/style/the_end_of_style_spec.rb

And a new require line is added to lib/rubocop.rb.

(snip)

require 'rubocop/cop/style/file_name'
require 'rubocop/cop/style/flip_flop'
require 'rubocop/cop/style/the_end_of_style' # New line

require 'rubocop/cop/rails/action_filter'

Actual Behaviour

A new requrie line is not added to lib/rubocop.rb.

(snip)

require 'rubocop/cop/style/file_name'
require 'rubocop/cop/style/flip_flop'

require 'rubocop/cop/rails/action_filter'

This is because there was a problem when adding to the end of existing department.

---

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).
• Used the same coding conventions as the rest of the project.
• 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.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

a cop

[koic]

Oops! I fixed it 💦