arity error for defspec

I spent a good deal of time trying to figure out why my tests in the form

```
(defspec foo 100
    (prop/for-all [] true)
    (prop/for-all [] false))
```

were passing.  I finally realized that defspec was only testing the first property, and silently ignoring any further arguments.  

If I had got an arity complaint it would have been obvious, so I refactored defspec to do that.