fix type hint
bump composer deps
add warning about schema to docs
hacky tests for graviton:generate:bundle command

Guess what type of test where not readily testable with Symfony\Component\Console\Tester\CommandTester.

If you guessed generator commands based on sension-generator-bundle you get a brownie point.

I'll need to find a better way to test these later on. I might seriously need to look into fsMock or the likes.