Bump tslint from 5.18.0 to 5.20.0 (#8)

Bumps [tslint](https://github.com/palantir/tslint) from 5.18.0 to 5.20.0.
- [Release notes](https://github.com/palantir/tslint/releases)
- [Changelog](https://github.com/palantir/tslint/blob/master/CHANGELOG.md)
- [Commits](palantir/tslint@5.18.0...5.20.0)

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