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.

.travis.yml

@@ -138,16 +138,6 @@ matrix:
       env: SWIGLANG=octave SWIGJOBS=-j2
       sudo: required
       dist: xenial
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.2 CPP11=1
-      sudo: required
-      dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.4 CPP11=1
-      sudo: required
-      dist: trusty
     - compiler: gcc
       os: linux
       env: SWIGLANG=perl5
@@ -328,6 +318,11 @@ matrix:
       env: SWIGLANG=java CPP11=1
       sudo: required
       dist: xenial
+    - compiler: gcc
+      os: linux
+      env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.4 CPP11=1
+      sudo: required
+      dist: trusty
     - os: linux
       env: SWIGLANG=python CPP11=1
       sudo: required
@@ -409,6 +404,9 @@ matrix:
     - compiler: clang
       os: osx
       env: SWIGLANG=lua
+    - compiler: clang
+      os: osx
+      env: SWIGLANG=octave SWIGJOBS=-j2
     - compiler: clang
       os: osx
       env: SWIGLANG=perl5
@@ -433,11 +431,9 @@ matrix:
       sudo: required
       dist: xenial
     # Sometimes hits the Travis 50 minute time limit
-    - compiler: gcc
+    - compiler: clang
-      os: linux
+      os: osx
-      env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.4 CPP11=1
+      env: SWIGLANG=octave SWIGJOBS=-j2
-      sudo: required
-      dist: trusty
     # Experimental languages
     - compiler: gcc
       os: linux

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