Bump underscore and gulp.spritesmith

Bumps [underscore](https://github.com/jashkenas/underscore) to 1.13.6 and updates ancestor dependency [gulp.spritesmith](https://github.com/twolfson/gulp.spritesmith). These dependencies need to be updated together.


Updates `underscore` from 1.4.4 to 1.13.6
- [Release notes](https://github.com/jashkenas/underscore/releases)
- [Commits](jashkenas/underscore@1.4.4...1.13.6)

Updates `gulp.spritesmith` from 6.11.0 to 6.13.0
- [Release notes](https://github.com/twolfson/gulp.spritesmith/releases)
- [Changelog](https://github.com/twolfson/gulp.spritesmith/blob/master/CHANGELOG.md)
- [Commits](twolfson/gulp.spritesmith@6.11.0...6.13.0)

---
updated-dependencies:
- dependency-name: underscore
  dependency-type: indirect
- dependency-name: gulp.spritesmith
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]