Don't register BulkChangeTable offense if block intervenes

[mvz]

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.
The Rails/BulkChangeTable cop currently won't suggest combining table change method calls if there is another method call between them. However, this logic fails if the intervening call has a block, like so:

add_column :users, :name, :string, null: false
User.find_each do |user|
  user.update(name: user.nick_name)
end
remove_column :users, :nickname

This pull request fixes that problem by looking at all of the method body's children, not just the :send nodes.

• 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]

Thanks!