Change ctrl-e to use nerdtree-tabs instead of nerd tree native

The original point of this line (which I think came from my config originally) was to do exactly what the nerdtree-tabs thing does; since we're using nerdtree tabs, seems like it makes more sense to make this change.  I think they do basically the same thing, but in case they don't (and to be clean) this uses the recommended way to toggle the nerdtree given by the nerdtree-tabs project.