specify osx_image to enable latest ruby on osx environment

.travis.yml

@@ -1,15 +1,47 @@
 language: ruby
 
-rvm:
-  - 2.1.10
-  - 2.2.4
-  - 2.3.0
-  - ruby-head
-  - rbx
+# http://rubies.travis-ci.org/
+# See here for osx_image -> OSX versions: https://docs.travis-ci.com/user/languages/objective-c
+matrix:
+  include:
+    - rvm: 2.1.10
+      os: linux
+    - rvm: 2.2.4
+      os: linux
+    - rvm: 2.3.0
+      os: linux
+    - rvm: ruby-head
+      os: linux
+    - rvm: 2.1.0
+      os: osx
+      osx_image: xcode7.3 # OSX 10.11
+    - rvm: 2.2.4
+      os: osx
+      osx_image: xcode7.1 # OSX 10.10
+    # - rvm: 2.3.0
+    #   os: osx
+    #   osx_image: .... # no valid version/env for ruby 2.3
+    - rvm: ruby-head
+      os: osx
+      osx_image: xcode 7.3 # OSX 10.11
+  allow_failures:
+    - rvm: ruby-head
+    - rvm: rbx
+    - rvm: 2.3.0
+      os: osx
+
+# rvm:
+#   - 2.1.10
+#   - 2.2.4
+#   - 2.3.0
+#   - ruby-head
+#   - rbx
+
+# os:
+#   - linux
+#   - osx
 
-os:
-  - linux
-  - osx
+# osx_image: xcode7.3
 
 branches:
   only:
@@ -25,13 +57,6 @@ script: bundle exec rake
 
 sudo: false
 
-matrix:
-  allow_failures:
-    - rvm: ruby-head
-    - rvm: rbx
-    - rvm: 2.3.0
-      os: osx
-
 addons:
   apt:
     packages: