Update Changelog for release of version 3.5.5

Author: LaunchDarklyCI

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to the LaunchDarkly PHP SDK will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org).
 
+## [3.5.5] - 2019-06-05
+### Fixed:
+- The SDK could throw an exception when calling `allFlagsState()` if APC/APCu caching was enabled. This bug was introduced in the 3.5.0 release. (Thanks, [omnicolor](https://github.com/launchdarkly/php-server-sdk/pull/124)!)
+- Improved unit test coverage for the caching logic.
+
 ## [3.5.4] - 2019-05-10
 ### Changed:
 - Changed the package name from `launchdarkly/launchdarkly-php` to `launchdarkly/server-sdk`