Support for PHP 7.1

[zinkovskiy]

Hello, @MewesK.
According composer.json of PHPExcel, it supports PHP7 and newer.
In my local machine I have installed php 7.1.5. I tried to run php unit tests of TwigExcelBundle with my version of php and it works fine (except some tests for XLS format, but I understood that it's normal: limited functionality as you wrote).
So, could you please change your composer requirements from "php": "5.5.9 - 7.0", to "php": "5.5.9 - 7.1", ?