PHP platform in composer.json root node is set incorrectly

[Majkl578]

#691 (comment)

a) it's invalid here, it must be in "config" node (has no effect here)
b) using 7.1.x is invalid, it must be a specific version (i.e. 7.1.3)
c) having this here will likely cause issues with SF4 since Migrations require ^7.1 and platform override therefore should be 7.1.0