Update PHP version dependency to ~7.2.0

[pmclain]

Description

Add PHP 7.2 to require of all composer.json files.

Fixed Issues (if relevant)

1. Update PHP version dependency to ~7.2.0 #15

Manual testing scenarios

1. Checkout branch or clone repository
2. Run composer install
3. Installation should fail, but not due to Magento's PHP version constraints.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[pmclain]

These changes are no longer current since PHP support settled on ~7.1.3||~7.2.0. Updating the version requires in #86.