Update composer.json to include version 4.0 for symfony libraries

[wesobi]

• Tested composer.json locally with minimum stability on beta, as it would otherwise only go up to 3.3.13

• Composer installed the packages, however I did not test it through the sandbox because the Liform is not up to date yet.

SymfonyConHackday2017

[nacmartin]

Ah! thanks @wesobi :) I will make a try with the Sandbox after updating Liform. If should be simple as well.

[wesobi]

@nacmartin no problem! Should indeed be simple to update it.