Replace `EnsureNode.body` with `EnsureNode.branch`

[dvandersluis]

EnsureNode.body has been deprecated in rubocop/rubocop-ast#337, and will be reimplemented in the next major version of rubocop-ast. In order to prepare for that, existing EnsureNode.body calls, which retreive the ensure branch of exception handling, have been replaced with EnsureNode.branch.

---

Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [ ] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [ ] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [ ] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[dvandersluis]

There appears to be an unrelated test failure:

  1) Failure:
MultipleAssertionsTest#test_all_types_of_assignments_are_understood [test/rubocop/cop/minitest/multiple_assertions_test.rb:131]:
--- expected
+++ actual
@@ -1,6 +1,6 @@
 "class FooTest < Minitest::Test
   def test_all_types_of_assignment
-  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test case has too many assertions [8/1].
+  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test case has too many assertions [9/1].
     # lvasgn
     foo = assert_equal(1, 1)
     # ivasgn

I opened #325 to fix it.