Docs: Include Pootle-specific settings naming rule in styleguide
Cleanup: Rename USE_API to POOTLE_ENABLE_API
Require Jinja2 < 2.7 for Python 2.5

Jinja2 is a requirement of Sphinx (used to build documentation), but the
latest (2.7) version requires Python 2.6; allow build to succeed by
installing older 2.6 version before processing requirements
Propagate failure status through |tail

This restores proper failure detection broken in earlier commit titled
"Reduce volume of Travis build output"
Add note about Jinja2 and Python 2.5