Don't generate post slugs for Ghost >= 0.8.0

[vickychijwani]

Starting with Ghost 0.8.0, the Ghost backend automatically updates the post slug to match the title, unless it's been changed manually or the post has been published at least once. This pretty much eliminates the need for our own slug generation logic.

So, this code should not be executed for >= 0.8.0 and removed once we stop supporting prior versions altogether.