Bump babel-plugin-tester from 6.2.1 to 6.4.0

Bumps [babel-plugin-tester](https://github.com/babel-utils/babel-plugin-tester) from 6.2.1 to 6.4.0.
- [Release notes](https://github.com/babel-utils/babel-plugin-tester/releases)
- [Changelog](https://github.com/babel-utils/babel-plugin-tester/blob/master/CHANGELOG.md)
- [Commits](babel-utils/babel-plugin-tester@v6.2.1...v6.4.0)

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