Bump npm from 5.8.0 to 6.13.4

Bumps [npm](https://github.com/npm/cli) from 5.8.0 to 6.13.4.
- [Release notes](https://github.com/npm/cli/releases)
- [Changelog](https://github.com/npm/cli/blob/latest/CHANGELOG.md)
- [Commits](npm/cli@v5.8.0...v6.13.4)

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

Author: dependabot[bot]