Require Xcode 7.1 to try and get NS_ASSUME_NONNULL_BEGIN
diff --git a/.travis.yml b/.travis.yml
index 3829104..c0d7c25 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,5 @@
 language: objective-c
+osx_image: xcode7.1
 env:
   global:
     - CONFIG=opt