Configure composer to assume PHP 7.1

[mlocati]

What about forcing the PHP version in the composer.json file? (ie config.platform.php = 7.1)

Since I think this tool will be mostly used as a phar, and since the phar must support the lowest php version, that makes sense imho...

[KorvinSzanto]

My comment here applies to this as well, I agree once phar is the only option we should lock it.