travis: ask for the xcode6.4 image

carlosmn · carlosmn · commit 2504d772432b · 2016-12-23T16:17:06.000Z
The newer image does not find mono 3.12 in PATH so our build fails with that.
diff --git a/.travis.yml b/.travis.yml
@@ -10,6 +10,9 @@ os:
  - osx
  - linux
 
+# The newer image cannot find mono 3.12 in the PATH
+osx_image: xcode6.4
+
 env:
  global:
   - MONO_OPTIONS=--debug
