Fix group_by test to work with jinja2 >= 2.9.

(cherry picked from commit cc3d131f50)

test/integration/test_group_by.yml

@@ -25,10 +25,10 @@
       group_by: key={{ genus }}
 
     - name: group by first three letters of genus with key in quotes
-      group_by: key="{{ genus | truncate(3, true, '') }}"
+      group_by: key="{{ genus[:3] }}"
 
     - name: group by first two letters of genus with key not in quotes
-      group_by: key={{ genus | truncate(2, true, '') }}
+      group_by: key={{ genus[:2] }}
 
     - name: group by genus in uppercase using complex args
       group_by: { key: "{{ genus | upper() }}" }