Add hook in travis to notify FB people when they break CI

Summary:
**what is the change?:**
Add a hook similar to facebook/relay@a1eff1a

**why make this change?:**
We want people who sync commits out from FB to be notified (and maybe
stopped?) if their change breaks CI on Github.

**test plan:**
Do it live

**issue:**
Follow up of facebookarchive#1364
Closes facebookarchive#1373

Differential Revision: D5802523

fbshipit-source-id: 795c29d865c6f0b36a89bb099c7092c9a96d56fd

.travis.yml

@@ -67,3 +67,6 @@ script:
     fi
 
   fi
+
+notifications:
+  webhooks: https://code.facebook.com/travis/webhook/