replace DataFrame.append with pd.concat to remove deprecation warning

Author: mortada

fredapi/fred.py

@@ -353,7 +353,7 @@ def __get_search_results(self, url, limit, order_by, sort_order, filter):
             for i in range(1, max_results_needed // self.max_results_per_request + 1):
                 offset = i * self.max_results_per_request
                 next_data, _ = self.__do_series_search(url + '&offset=' + str(offset))
-                data = data.append(next_data)
+                data = pd.concat([data, next_data])
         return data.head(max_results_needed)
 
     def search(self, text, limit=1000, order_by=None, sort_order=None, filter=None):

fredapi/version.py

@@ -1,2 +1,2 @@
 
-version = '0.5.0'
+version = '0.5.1'