Bump version of sass.

The older version was causing the following error with ruby 2.1:

```
Mixin "ie-lte" does not accept a content block.
  (in
  /var/govuk/design-principles/app/assets/stylesheets/application-ie6.css.scss)
  /var/govuk/design-principles/app/assets/stylesheets/main.css.scss:1
  /var/govuk/design-principles/app/assets/stylesheets/application.css.scss:8
  /var/govuk/design-principles/app/assets/stylesheets/application-ie6.css.scss:6
```