Fix Travis testing for 2.5.0 release, document Jinja2/Python 2.5 issue

A new release of Jinja2 (2.7) came out yesterday, and it no longer supports Python 2.5, requiring 2.6 or later.  Since the Sphinx documentation tool is needed for development builds (and Travis testing) and it requires Jinja2, when using Python 2.5 you have to explicitly install an earlier version of Jinja2 before installing the Pootle development requirements.

Also included in this PR are some additional Travis testing for the stable/2.5.0 branch, as well as related fixes allowing testing with earliest supported Django version (1.3.0) and reducing useless output during the Travis build process (while properly detecting failures).