osx_image: xcode7

http://blog.travis-ci.com/2015-08-26-xcode-70-beta-is-now-available/

.travis.yml

@@ -1,5 +1,5 @@
 language: objective-c
-osx_image: xcode6.4
+osx_image: xcode7
 cache:
   directories:
   - vendor/bundle