Intermittent failure in email throttling test

```
  1) Emails Throttling verify email throttling:
     Uncaught AssertionError: null == true
      at /home/travis/build/oaeproject/Hilary/node_modules/oae-email/tests/test-email.js:625:40
      at EventEmitter._handleDebugSent (/home/travis/build/oaeproject/Hilary/node_modules/oae-email/lib/test/util.js:49:32)
      at EventEmitter.g (events.js:180:16)
      at EventEmitter.emit (events.js:95:17)
      at /home/travis/build/oaeproject/Hilary/node_modules/oae-email/lib/api.js:9:16542
      at /home/travis/build/oaeproject/Hilary/node_modules/redback/lib/advanced_structures/RateLimit.js:99:9
      at /home/travis/build/oaeproject/Hilary/node_modules/redis/index.js:1136:13
      at b (domain.js:183:18)
      at try_callback (/home/travis/build/oaeproject/Hilary/node_modules/redis/index.js:573:9)
      at RedisClient.return_reply (/home/travis/build/oaeproject/Hilary/node_modules/redis/index.js:661:13)
```