Fix Travis testing for master, document Jinja2/Python 2.5 issue

[This is the PR for cherry-picks of [PR#91](https://github.com/translate/pootle/pull/91) commits onto master branch)

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 is a (cherry-picked) fix for Travis testing for master tp properly detect failures when suppressing excessive output.