[sass] Tell, that we use legacy api

[bato3]

Partial-fix #452

This patch only fixes the most pressing problem.

There is a question of how to deal with libraries that use legacy importers.

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Tests

• Run the tests with npm test or yarn test

[dummdidumm]

The different interface import seems to have broken some tests

[kaisermann]

This will be merged and used in #562. I actually tried to move to the new compileString API, but couldn't make it work in the testing environment unfortunately.