Remove deprecated argument in GHME 2013 notebook

The `order` keyword wasn't actually used by `factorize`. See
pandas-dev/pandas#6926.

Author: kyleam

pymc/examples/GHME 2013.ipynb

@@ -49,7 +49,7 @@
       "\n",
       "age = data['age'] = np.array(data.age_start + data.age_end)/2\n",
       "rate = data.value = data.value*1000\n",
-      "group, countries = pd.factorize(data.area, order=countries)\n",
+      "group, countries = pd.factorize(data.area)\n",
       "\n",
       "\n",
       "ncountries = len(countries)"