Octave: drop support for Octave versions older than 3.2.0

- Latest non-supported version is 3.0.5, released 2009-04-09
- Earliest supported version is 3.2.0, released 2009-06-05
.travis.yml: also test against Octave version 3.8
DO-NOT-PUSH .travis.yml: test against Octave versions 3.2, 3.4, 3.6, 3.8