Build assets as part of CI job.

This will allow us to catch any errors early.
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
```