Update symfony-storage requirement

[odolbeau]

Fix #373

[odolbeau]

It looks like phpstan is ran against latest vendors version (master) due to "minimum-stability": "dev" in our composer.json.

Should we:

1. Keep running phpstan against non stable version & remove this error message?
2. Keep "minimum-stability": "dev" & find a way to run phpstan against latest stable vendor versions? (maybe using docker://jakzal/phpqa:php7.3-alpine instead of docker://oskarstark/phpstan-ga)
3. Remove "minimum-stability": "dev" from our composer.json?

I'll create a new PR to try solution 2.

[odolbeau]

Changing the docker image is not enough to have solution 2. working (see #375 )

[Nyholm]

Thank you.

It should be green now when #375 is merged.