Travis Octave testing changes

- 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.

Tools/travis-osx-install.sh

@@ -22,6 +22,9 @@ case "$SWIGLANG" in
 	"lua")
 		travis_retry brew install lua
 		;;
+	"octave")
+		travis_retry brew install octave
+		;;
 	"python")
 		WITHLANG=$SWIGLANG$PY3
 		if [[ "$PY3" ]]; then