Bump babel-plugin-istanbul from 4.1.5 to 5.1.1

Bumps [babel-plugin-istanbul](https://github.com/istanbuljs/babel-plugin-istanbul) from 4.1.5 to 5.1.1.
- [Release notes](https://github.com/istanbuljs/babel-plugin-istanbul/releases)
- [Changelog](https://github.com/istanbuljs/babel-plugin-istanbul/blob/master/CHANGELOG.md)
- [Commits](istanbuljs/babel-plugin-istanbul@v4.1.5...v5.1.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support