Bump backbone from 1.3.3 to 1.4.0

Bumps [backbone](https://github.com/jashkenas/backbone) from 1.3.3 to 1.4.0.
- [Release notes](https://github.com/jashkenas/backbone/releases)
- [Commits](https://github.com/jashkenas/backbone/compare/1.3.3...1.4.0

)

Signed-off-by: dependabot[bot] <support@dependabot.com>
Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>