- Keep testing Octave 4.0 on Xenial, but the Octave headers break the C++11
tests, so just test C++98.
- Drop Octave 4.2 testing on Trusty.
- Keep Octave 4.4 testing on Trusty - it is the only C++11 Octave testing
done and is just fast enough to beat the 50 minute limit.
- Add latest Octave version (5.1) testing by using MacOS and brew. It takes
about 49 minutes to run for C++98 testing.
Unfortunately it takes longer than the Travis 50 minute limit to test C++11.
I can't find a Xenial or MacOS solution for testing Octave and C++11, so we'll
keep it on Travis for now. Octave is available as a Flatpak's, but Flatpak
doesn't work on Travis as it requires a reboot after installing the flatpak
package.
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