Update changelog

CHANGELOG.md

@@ -1,13 +1,19 @@
 Changelog
 =========
 
-## TBD
+## 6.15.0 (27 July 2020)
 
 ### Enhancements
 
 * Add `on_error` callbacks to replace `before_notify_callbacks`
   | [#608](https://github.com/bugsnag/bugsnag-ruby/pull/608)
 
+* Improve performance when extracting code from files in stacktraces
+  | [#604](https://github.com/bugsnag/bugsnag-ruby/pull/604)
+
+* Reduce memory use when session tracking is disabled
+  | [#606](https://github.com/bugsnag/bugsnag-ruby/pull/606)
+
 ### Deprecated
 
 * `before_notify_callbacks` have been deprecated in favour of `on_error` and will be removed in the next major release