Let's try adding OS X to the Travis file.

.travis.yml

@@ -1,11 +1,14 @@
+os:
+  - linux
+  - osx
 
 # This makes us use Ubuntu 14 instead of 12
 dist: trusty
 
-language: cpp
+language: c
 script:
-  - sudo apt-get update
-  - sudo apt-get install libgtk-3-dev -y || sudo apt-cache search libgtk3
+  - [ Z$TRAVIS_OS_NAME = Zlinux ] && sudo apt-get update
+  - [ Z$TRAVIS_OS_NAME = Zlinux ] && { sudo apt-get install libgtk-3-dev -y || sudo apt-cache search libgtk3; }
   - make -f GNUmakefile
   - make -f GNUmakefile examples
   - make -f GNUmakefile test