BC breaks with PHP v5.5 and v5.6

[ryanolee]

Issue

Changes in recent version of react renderer break backwards compatibility with PHP versions 5.x. The composer.json file references a minimum compatibility with PHP 5.5 (https://github.com/Limenius/ReactRenderer/blob/e1bf24deb09f1009fdec7e90cf29378220351fe7/composer.json#L22) However there are a number of changes in the repo that do not conform to valid PHP 5 syntax. (Using features that were only added in PHP v7+)

Versions effected

• 5.*
• 4.*

How to recreate

Install package in any version of php5 and follow a normal use case

How we might fix it

• Bump the minimum PHP requirement to 7 in v5
• Remove all changes that caused BC breaks for php 5 in v4