Bump sass from 3.5.5 to 3.5.6
Bump sass from 3.5.5 to 3.5.6

Bumps [sass](https://github.com/sass/sass) from 3.5.5 to 3.5.6.
- [Release notes](https://github.com/sass/sass/releases)
- [Commits](https://github.com/sass/sass/commits)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall