Jinja2 release 2.9.2 breaks several pages in webserver

[Erich Hochmuth]

It appears the Jinja2 (2.9.2) release with Python 2.7.X breaks several
pages in the 1.7.3 release of airflow. The Charts and Known issues fail to
load. Downgrading to Jinja2 2.8.1 resolves the issues. I've attached a
screen shot of the error.