Add The Ability To Use The Cache To Prevent Race Conditions on Indexes

I've been using this on a site I have that generates models rapidly and found that Eloquent-Sluggable would use duplicate indexes for slugs if generated quickly. It's not so much a problem with the library but more a fact that looking up indexes in a database can be a bit slow. This version of the library will now look up the index in the database and also consult the cache so that an index can be locked.

It's a bit raw right now as I needed a quick fix for my project, but it might be helpful to someone. Disregard if not.