move travis-ci to macOS

.travis.yml

-language: python
+language: cpp
+sudo: required
+os: osx
+osx_image: xcode7.3
+cache: ccache
 
-install:
-  - export
-  - sudo add-apt-repository ppa:octave/stable -y
-  - sudo apt-get update -qq
-  - sudo apt-get install -qq octave liboctave-dev
+cache:
+  directories:
+  - /usr/local
+
+before_install:
+  - brew update
+  - brew tap homebrew/science
+  - brew install ccache
+  - brew install octave
 
 script:
   - make check-octave