Reduce the Travis CI test matrix

- don't test old Node.JS version on OSX
- don't test iojs, should be covered by 0.12 and 4

.travis.yml

@@ -20,6 +20,12 @@ env:
 
 matrix:
   fast_finish: true
+  exclude:
+    - node_js: iojs
+    - node_js: "0.10"
+      os: osx
+    - node_js: "0.12"
+      os: osx
 
 addons:
   apt: