[5.0] Use Doctrine Inflector

We actually already require people to download this package with laravel anyway because it is a dependency of the doctrine annotations package, so we might as well start using it. This can only be an advantage as far as I can see. It's a really small package, and takes the weight of maintaining the pluraliser off laravel.

NB, we used to have a second parameter in the pluralise function in laravel - I checked, and the laravel core doesn't use that parameter at all. As Laravel 5.0 is breaking, I don't see an issue with removing that param.
I can add back this functionality in the str class if you like.