Update Changelog for release of version 5.0.0

Author: LaunchDarklyCI

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 All notable changes to the LaunchDarkly Ruby SDK will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org).
 
+## [5.0.0] - 2018-06-26
+### Changed:
+- The client no longer uses Celluloid for streaming I/O. Instead, it uses [socketry](https://github.com/socketry/socketry).
+- The client now treats most HTTP 4xx errors as unrecoverable: that is, after receiving such an error, it will not make any more HTTP requests for the lifetime of the client instance, in effect taking the client offline. This is because such errors indicate either a configuration problem (invalid SDK key) or a bug, which is not likely to resolve without a restart or an upgrade. This does not apply if the error is 400, 408, 429, or any 5xx error.
+- During initialization, if the client receives any of the unrecoverable errors described above, the client constructor will return immediately; previously it would continue waiting until a timeout. The `initialized?` method will return false in this case.
+
+### Removed:
+- The SDK no longer supports Ruby versions below 2.2.6, or JRuby below 9.1.16.
+
 ## [4.0.0] - 2018-05-10
 
 ### Changed: