[Fix #157] Add new `Minitest/EmptyLinesBeforeAssertionMethods` cop

[koic]

Fixes #157.

This PR adds new Minitest/EmptyLinesBeforeAssertionMethods cop.

This cop enforces empty line before assertion method.

# bad
do_something
assert_equal(expected, actual)

# good
do_something

assert_equal(expected, actual)

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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.
• [x] 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.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] 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.