Update travis's xcode version to 6.4
diff --git a/.travis.yml b/.travis.yml
index e72b83f..c1d84df 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,5 @@
 language: objective-c
-osx_image: xcode63
+osx_image: xcode6.4
 
 before_install:
 - gem install xcpretty -N