evaluate phpcompatability

[ofbeaton]

phpcompatibility/php-compatibility is a neat package that provides testing your code against language features. You can specify a specific language version --runtime-set testVersion 5.0 or that version and up --runtime-set testVersion 5.0- or a specific range --runtime-set testVersion 5.3-5.6.

This would be incredibly useful. Right now they want us to use installed_paths to install it, which isn't ideal for us. Upstream ticket to see if they'll make it so we can use traditional ruleset inclusion.

PHPCompatibility/PHPCompatibility#793