Mark iOS tests as able to fail.

Travis updated their images to include an xctool that can randomly kill
tests, so mark them as flaky to avoid turning things red.
diff --git a/.travis.yml b/.travis.yml
index 9bc4d88..60c0427 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -71,5 +71,14 @@
     # we moved to an OS X image that is 10.11.
     - os: osx
       env: CONFIG=python_cpp
+    # xctool 0.2.8 seems to have a bug where it randomly kills tests saying
+    # they failed.
+    #   https://github.com/facebook/xctool/issues/619
+    #   https://github.com/google/protobuf/issues/1232
+    # travis updated their images to include 0.2.8:
+    #   https://blog.travis-ci.com/2016-03-23-xcode-image-updates
+    # Mark the iOS test as flakey so these failures don't turn things red.
+    - os: osx
+      env: CONFIG=objectivec_ios
 notifications:
   email: false