PHP-SDK - 2.2.1

### Bug fixes
- fix(generateBucketValue): Avoid negative bucket number for PHP x86. ([#137](https://github.com/optimizely/php-sdk/pull/137))
- fix(phpdoc-notification-callback): Fixes phpdoc primitive type of notification-callback. ([#135](https://github.com/optimizely/php-sdk/pull/135))

https://github.com/optimizely/php-sdk/releases/tag/2.2.1