fix jinja2 build error with old version of flask by pinning jinja2 (#517)

Author: imatiach-msft

requirements-test.txt

@@ -13,4 +13,5 @@ itsdangerous==2.0.1
 markupsafe<2.1.0
 # Jupyter dependency that fails with python 3.6
 pywinpty==2.0.2; python_version <= '3.6' and sys_platform == 'win32'
-jupyter
+jupyter
+jinja2==2.11.3