Ah, I guess when you take the Hammock localization of the category of simplicial categories, with the Bergner model structure, you get a simplicial category, but not a simplicial model category. But that's okay, because (I think?) the simplicial nerve of this simplicial category will be equivalent to the "$\infty$-category of $\infty$-categories," since the category of simplicial sets with the Bergner model structure is Quillen equivalent to simplicial sets with the Joyal model structure.