Require Jinja2>=2.6

The Designate project requires Jinja2 functionality available only in
version 2.6+.

Additionally, add a comment specifying the Jinja2 license (BSD 3 Clause)

Partial-Bug: 1375613
Change-Id: I4cf8a38106385990b278475d6efc43245461ba6e
Merge "Require Jinja2>=2.6"