[CI] Mark Octave 4.2 build as "allowed to fail"

It's started to reliably fail at package install time.  See #909.

.travis.yml

@@ -85,11 +85,6 @@ matrix:
       env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.0
       sudo: required
       dist: trusty
-    - compiler: gcc
-      os: linux
-      env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.2 CPP11=1
-      sudo: required
-      dist: trusty
     - compiler: gcc
       os: linux
       env: SWIGLANG=perl5
@@ -300,6 +295,12 @@ matrix:
       env: SWIGLANG=python SWIG_FEATURES=-O
       sudo: required
       dist: trusty
+    # Has started to fail at package install time
+    - compiler: gcc
+      os: linux
+      env: SWIGLANG=octave SWIGJOBS=-j2 VER=4.2 CPP11=1
+      sudo: required
+      dist: trusty
 before_install:
   - date -u
   - uname -a