Use Xcode 7.1 in Travis CI
diff --git a/.travis.yml b/.travis.yml
index 0bad31e..05cfcb6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,2 +1,3 @@
 language: objective-c
 script: ./Run-Tests.sh
+osx_image: xcode7.1