add the osx build (xcode7.3, brew install cairo)

.travis.yml

@@ -1,4 +1,11 @@
 language: node_js
+os:
+  - linux
+  - osx
+osx_image: xcode7.3 # has everything we need but cairo
+cache:
+  directories:
+    - ~/HomebrewCache
 node_js:
   - '6'
   - '4'
@@ -18,6 +25,7 @@ addons:
 env:
   - CXX=g++-4.9
 before_install:
+  - if [[ $TRAVIS_OS_NAME == 'osx' ]]; then export HOMEBREW_CACHE=~/HomebrewCache; brew update && brew install cairo; fi
   - if [[ $TRAVIS_NODE_VERSION == 0.8 ]]; then npm install -g npm@1.4.28; fi
   - npm explore npm -g -- npm install node-gyp@latest
 sudo: false