Requiring PHP 7.1

[guilhermednt]

The much awaited upgrade to PHP 7.1 is here!

In the next months we'll be upgrading to Symfony 4.1 as well, so stay tuned.

Close PROCERGS#785

Tasks:

• [x] Run composer update
• [x] Update dependencies
• [x] Check Deprecations
• [x] Check if everything looks OK