Changes required PHP version in readme

[netorodrigues]

Questions	Answers
What?	Updates minimum PHP required version.
Why?	To avoid misunderstanding once all other sources, like this wiki and the official documentation, points 7.1 as minimum.
How?	Updating project readme.

:package: Attachments (if appropriate)

Add additional informations like screenshots, issue link, etc

:speech_balloon: Important guidelines

• Add pull request labels.
• Check base branch.
• Make sure your local branch is up to date before commiting your changes!