Accept newer versions of Jinja2

[lorenzogil]

I've tested glue with Jinja2 2.8 and it works perfectly. Both with unittests and manual testing generating a sprite from the famfam icon set.

Jinja2 2.8 was released almost a year ago and most recent Linux distributions use it by default.

[153957]

Already covered by 37aecf52db6f5fd52cdb9e288fde80fa45a37c19