Require Jinja2 version with tojson

Conengmo · Conengmo · commit db8b1df4ddb3 · 2019-04-07T11:31:52.000+02:00
The `tojson` filter was introduced in version 2.9.
diff --git a/requirements.txt b/requirements.txt
@@ -1,4 +1,4 @@
 branca>=0.3.0
-jinja2
+jinja2>=2.9
 numpy
 requests
