Updated Kubeflow stable and latest versions (#933)

* Changed Kubeflow stable version to v0.5.1.

* Updated the link ref too.

* Removed hyperlink as it's not useful.

* Added shortcode for kf latest version.

* Added line breaks at end of file.

layouts/shortcodes/kf-latest-version.html

+v0.6.0-rc.0

layouts/shortcodes/kf-stable-tag.html

-<a href="https://github.com/kubeflow/kubeflow/releases/tag/v0.4.1">v0.4.1</a>
+v0.5.1