Merge pull request #343 from surfly/travis

drop support for 2.5; improve travis; use py2.6+ syntax
travis: disable email notifications
travis: replace ack with grep

since ack is not available this caused

  ack -w subprocess greentest/ -l -v | python -c 'import sys; print "\n".join(line.split("/")[-1].strip() for line in sys.stdin)' > greentest/tests_that_dont_use_subprocess.txt

to fail and generate empty tests_that_dont_use_subprocess.txt, but due to a pipe (|), it still had exit code 0 and did not abort the build.