Cleanup specs

* use described_class
* do not use instance variables
* do not start example description with should
[Fixes #1094] Fix bug in ClassAndModuleChildren

ClassAndModuleChildren incorrectly flaged class with a single method
Merge pull request #1095 from geniou/rspec_cleanup

Cleanup specs
Merge pull request #1096 from geniou/class_and_module_children_single_method

[Fixes #1094] Fix bug in ClassAndModuleChildren
Fix incorrect error message about rubocop-todo.yml
Merge pull request #1098 from agrimm/todo_filename

Fix incorrect error message about rubocop-todo.yml