try osx on travis

Author: h-2

.travis.yml

@@ -26,6 +26,13 @@ matrix:
           sources: ['ubuntu-toolchain-r-test']
           packages: ['clang', 'g++-4.9', 'cmake', 'zlib1g-dev', 'libbz2-dev'] # g++ required for newer libstdc++
 
+    - os: osx
+      compiler: clang-osx
+      before_install:
+        - sudo brew update
+        - sudo brew tap homebrew/versions
+        - sudo brew install cmake
+
 script:
   - sudo dd if=/dev/zero of=/swapfile bs=64M count=32 # cheat us some extra MEM
   - sudo mkswap /swapfile