Modify Travis testing to reflect Pootle 2.5.1 requirements:

Python 2.5 - 2.7
Django 1.4 - 1.4.5

also add some forward looking excludes to the matrix
always notify on failure (on master & release branches, at least)
Python 2.5 relative import * fix

For some reason 2.5 is fussier about relative imports than later versions
Use absolute imports instead to keep it happy
Document Django requirement for Pootle 2.5.1 in changelog
Document VCS_DIRECTORY and split in changelog for 2.5.0
Documentation grammar fix