travis: enable OS X tests

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index b1e0444..6fe85fc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,6 +7,10 @@
 
 sudo: false
 
+os:
+  - linux
+  - osx
+
 addons:
   ssh_known_hosts:
     - bitbucket.org