Version numbers use h3

Author: mwouts

docs/source/summary-changes.md

@@ -3,7 +3,7 @@
 See `git log` for a more detailed summary of changes.
 ## 2.4
 
-## 2.4.0 (2019-08-11)
+### 2.4.0 (2019-08-11)
 
 * [PR-323](https://github.com/jupyter/kernel_gateway/pull/323): Update handler not use deprecated maybe_future call
 * [PR-322](https://github.com/jupyter/kernel_gateway/pull/322): Update handler compatibility with tornado/pyzmq updates