Tell friendly_id to regenerate slug if title has changed.

This is due change in friendly_id 5.0.x where it doesn’t regenerate slug when record is saved.

Directly from https://github.com/norman/friendly_id readme:
> Slugs are no longer regenerated when a record is saved. If you want to regenerate a slug, you must explicitly set the slug column to nil.

> You can restore some of the old behavior by overriding the should_generate_new_friendly_id? method.