Add basic Sass test to check our Sass compiles

Tested by reverting 31b973d11c968a5db5a4e4de5a19508eac932333 and running
the tests. They failed with:

```
 > grunt
Running "sass:development" (sass) task
Syntax error: Positional arguments must come before keyword arguments.
        on line 42 of /Users/jabley/govuk/govuk_frontend_toolkit/stylesheets/_typography.scss
        from line 2 of /Users/jabley/govuk/govuk_frontend_toolkit/stylesheets/design-patterns/_alpha-beta.scss
        from line 6 of spec/stylesheets/test.scss
  Use --trace for backtrace.
Warning: Exited with error code 1 Use --force to continue.

Aborted due to warnings.
```
Merge pull request #125 from alphagov/feature/sass-tests

Add basic Sass test to check our Sass compiles