Use Travis CI's multi-OS feature

.travis.yml

@@ -4,6 +4,9 @@ node_js:
   - "0.12"
   - "4"
   - "5"
+os:
+  - linux
+  - osx
 sudo: false
 addons:
   apt:
@@ -11,7 +14,9 @@ addons:
     - ubuntu-toolchain-r-test
     packages:
     - g++-4.8
-env:
-  CXX=g++-4.8
+osx_image: xcode7.3
+before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then export CXX=g++-4.8; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install homebrew/science/vips; fi
 after_success:
   - cat ./coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js