Fix RuboCop offenses in rails_helper.rb template

[kengallego]

Rubocop:

Rails/FilePath: Prefer Rails.root.join('path/to').
Lint/RedundantDirGlobSort: Remove redundant sort.

[pirj]

For the reference of join: rubocop/rubocop-rails#128

Regarding redundant sort, are you confident about this?

  #   This cop is unsafe, in case of having a file and a directory with
  #   identical names, since directory will be loaded before the file, which
  #   will break `exe/files.rb` that rely on `exe.rb` file.

[kengallego]

@pirj Good point about the sort, I already added it again. And thank you for the reference.

[pirj]

Thank you!