Change version to 0.2.1

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What's Changed

Changed

• Require Ruby 2.7 by @r7kamura in https://github.com/r7kamura/rubocop-slim/pull/6

  Fixed

• Fix error that occurs when when clause has multiple elements by @r7kamura in https://github.com/r7kamura/rubocop-slim/pull/5
• Fix Rubocop::VERSION to RuboCop::VERSION by @r7kamura in https://github.com/r7kamura/rubocop-slim/pull/7

  Others

• Rename RubyClipper to KeywordRemover by @r7kamura in https://github.com/r7kamura/rubocop-slim/pull/3
• Use sevencop by @r7kamura in https://github.com/r7kamura/rubocop-slim/pull/4

Full Changelog: https://github.com/r7kamura/rubocop-slim/compare/v0.2.0...v0.2.1