Add --ignore-platform-reqs to composer install

[BronsonQuick]

Without this users can end up with messages such as: - doctrine/annotations v1.6.0 requires php ^7.1 -> your PHP version (7.0.31) does not satisfy that requirement.

[rmccue]

Wouldn't you want the platform reqs message though?

[BronsonQuick]

@rmccue I've seen quite a few devs needing to pass this option in to get things working. Reckon I should add that as an yaml option instead? So default would be composer install and optionally pass a yaml value to add --ignore-platform-reqs? If so I'll open a new issue for that.

[rmccue]

That's usually an indication that their Chassis box needs to be reconfigured; e.g. with the error above, they should switch to PHP 7.1.

Generally, you want to do --ignore-platform-reqs if you're building on a non-functional system. If you're running Composer on your host (i.e. macOS), you want to ignore them, because the development you're installing for is Chassis instead. But, inside the box, they shouldn't be ignored.