Enable OS X Travis tests

.travis.yml

@@ -1,16 +1,14 @@
 os:
   - linux
+  - osx
 
 # This makes us use Ubuntu 14 instead of 12
 dist: trusty
 
 language: c
 script:
-  - sudo apt-get update
-  - sudo apt-get install libgtk-3-dev -y || sudo apt-cache search libgtk3
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get update; fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get install libgtk-3-dev -y || sudo apt-cache search libgtk3; fi
   - make -f GNUmakefile
   - make -f GNUmakefile test
   - make -f GNUmakefile examples 
-
-# TODO osx
-# need to figure out how to force language: objective-c and turn off the apt-get rules