Replaced render with render_to_string

Mojolicious 5 replaced the partial rendering using the `render` helper
with a `render_to_string` helper.

This commit updates the plugin to reflect that change.

For more information, see this post by sri:
https://groups.google.com/d/msg/mojolicious/PYfjHyVj26w/jrWH33bcuV4J
Merge pull request #2 from oliverguenther/master

Replaced render with render_to_string