Added the release version

Author: agronholm

docs/versionhistory.rst

@@ -4,7 +4,7 @@ Version history
 To find out how to migrate your application from a previous version of
 APScheduler, see the :doc:`migration section <migration>`.
 
-**UNRELEASED**
+**4.0.0a5**
 
 - **BREAKING** Added the ``cleanup()`` scheduler method and a configuration option
   (``cleanup_interval``). A corresponding abstract method was added to the ``DataStore``