Fix faulty logic in __call__ method of Runner class, fixes fastai#370 (…

…fastai#372)

* Fix faulty logic in __call__ method of Runner class, fixes fastai#370

Change `res = cb(cb_name) and res` to `res = cb(cb_name) or res`
so that `__call__` method does not always return `False` but returns
`True` if at least one of the callbacks returns `True`.

* Strip changed notebook 05b_early_stopping

nbs/dl2/05b_early_stopping.ipynb

@@ -187,7 +187,7 @@
     "\n",
     "    def __call__(self, cb_name):\n",
     "        res = False\n",
-    "        for cb in sorted(self.cbs, key=lambda x: x._order): res = cb(cb_name) and res\n",
+    "        for cb in sorted(self.cbs, key=lambda x: x._order): res = cb(cb_name) or res\n",
     "        return res"
    ]
   },