Bump npm from 6.11.3 to 6.13.4

Bumps [npm](https://github.com/npm/cli) from 6.11.3 to 6.13.4.
- [Release notes](https://github.com/npm/cli/releases)
- [Changelog](https://github.com/npm/cli/blob/latest/CHANGELOG.md)
- [Commits](https://github.com/npm/cli/compare/v6.11.3...v6.13.4)

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