Updated change log for release

Author: kmvanbrunt

CHANGELOG.md

@@ -1,7 +1,11 @@
-## 1.3.4 (TBD)
+## 1.3.4 (August 20, 2020)
 * Bug Fixes
     * Fixed `AttributeError` when `CommandSet` that uses `as_subcommand_to` decorator is loaded during
     `cmd2.Cmd.__init__()`.
+* Enhancements
+    * Improved exception messages when using mock without `spec=True`.
+    See [testing](https://cmd2.readthedocs.io/en/latest/testing.html) documentation for more details on testing
+    cmd2-based applications with mock.
 
 ## 1.3.3 (August 13, 2020)
 * Breaking changes