Update to Stripe PHP API 7.x / Drop support for PHP 5.4 & 5.5

[pderksen]

Do in parallel with Pro version.

• In a minor release leading up to a release with this, check for PHP < 5.6 and add non-dismissable admin notice. Add notice about PHP 5.6 requirement coming soon #58
• In this release itself, gracefully halt plugin and show admin notice.

[drzraf]

• There is no 7.x release : https://packagist.org/packages/stripe/stripe-php
• stripe-php 6.x supports PHP >= 5.4.0.
• WP 5.0 supports PHP >= 5.2.4
• => Unless wp-simplepay itself uses > 5.4 there is no reason to higher version requirements.

[pderksen]

@drzraf You're correct in that it's not in master yet, but it will be most likely a part of the 7.x release when that becomes current. https://github.com/stripe/stripe-php/tree/integration-v7

[spencerfinnell]

A notice to upgrade to PHP 5.6 will be shown in version 2.1. Once the Stripe PHP API library is updated to 7.x the plugin will require that as a minimum to function: #71