travis: add OS X continous integration

The travis guys were so nice to activate multi OS support for us (it's a beta
feature). So now we build on OS X ass well to check for OS X specific breakage.

Later I might investigate further and build with the minimum supported SDK
version so that we don't break older systems by using newer Cocoa features.

travis-deps

@@ -89,8 +89,15 @@ end
 
 class Dependencies < TravisDepsBuilder
   def deps
-    sh "sudo apt-get update -y"
-    sh "sudo apt-get install pkg-config fontconfig libfribidi-dev yasm -y"
+    case ENV['TRAVIS_OS_NAME']
+    when 'linux' then
+      sh "sudo apt-get update -y"
+      sh "sudo apt-get install pkg-config fontconfig libfribidi-dev yasm -y"
+    when 'osx' then
+      sh 'brew install pkg-config fontconfig freetype fribidi yasm'
+    else
+      abort "Unrecognized OS"
+    end
   end
 end