Cost Calculator for WordPress Version History

* [#920](https://github.com/stripe/stripe-php/pull/920) Add support for the `Session` resource and APIs on the `BillingPortal` namespace

Major version release. List of backwards incompatible changes to watch out for:

-   The minimum PHP version is now 5.4.0. If you're using PHP 5.3 or older, consider upgrading to a more recent version.

*   `\Stripe\AttachedObject` no longer exists. Attributes that used to be instances of `\Stripe\AttachedObject` (such as `metadata`) are now instances of `\Stripe\StripeObject`.

-   Attributes that used to be PHP arrays (such as `legal_entity->additional_owners` on `\Stripe\Account` instances) are now instances of `\Stripe\StripeObject`, except when they are empty. `\Stripe\StripeObject` has array semantics so this should not be an issue unless you are actively checking types.

*   `\Stripe\Collection` now derives from `\Stripe\StripeObject` rather than from `\Stripe\ApiResource`.

Pull requests included in this release:

-   [#410](https://github.com/stripe/stripe-php/pull/410) Drop support for PHP 5.3
-   [#411](https://github.com/stripe/stripe-php/pull/411) Use traits for common API operations
-   [#414](https://github.com/stripe/stripe-php/pull/414) Use short array syntax
-   [#404](https://github.com/stripe/stripe-php/pull/404) Fix serialization logic
-   [#417](https://github.com/stripe/stripe-php/pull/417) Remove `ExternalAccount` class
-   [#418](https://github.com/stripe/stripe-php/pull/418) Increase test coverage
-   [#421](https://github.com/stripe/stripe-php/pull/421) Update CA bundle and add script for future updates
-   [#422](https://github.com/stripe/stripe-php/pull/422) Use vendored CA bundle for all requests
-   [#428](https://github.com/stripe/stripe-php/pull/428) Support for automatic request retries