Remove PHP requirements from composer.json

[tmotyl]

It doesn't make sense to add PHP requirements in composer.json as they are now: (php 5.5, 5.6, 7.0, 7.1)

• this configuration prevents from using the package in PHP 7.2
• Magento 2 doesnt support php 5.5. or 5.6, so it doesnt make sense to specify it here
• This is M2 module, so we should rely on magento PHP requirements
  for every new PHP version this line needs to be updated

[amenk]

Please merge this :-) Is it abandoned ?