Add development dependency for SASS

Need in development for testing. In production, this will already be
included
Adjust processors test to use processors directly
Require sass gem in processors test

So Tilt doesn’t complain
Use same content for all tests

This change means testing sass stylesheet doesn’t actually do the sass
compilation step. For now, this will have to do.
Adjust dummy app

Put `.scss` extension in `<link>` tag for external sass stylesheets
`get_contents` methods directly use `context.evaluate`