The .travis.yml system allows plugins to use a .travis.yml file that is always in sync w/ core's .travis.yml. Plugins can customize the behavior of the file in two ways: by creating additional commands to execute or by setting environment variables to control the behaviour of travis scripts (ie, the scripts in tests/travis). The environment variables, such as $TEST_AGAINST_PIWIK_BRANCH, $TEST_AGAINST_CORE, $UNPROTECTED_ARTIFACTS and more, are not currently documented. They should be documented.
Added documentation to http://developer.piwik.org/guides/tests-travis.