Intermittent test failure: previews embeddable check

```
[0m  1) Preview processor Preview generation verify default link processing checks if a url is embeddable in an iframe:
[0m[31m     Uncaught AssertionError: false == true[0m[90m
      at /home/travis/build/oaeproject/Hilary/node_modules/oae-preview-processor/tests/test-previews.js:700:24
      at /home/travis/build/oaeproject/Hilary/node_modules/oae-preview-processor/tests/test-previews.js:302:33
      at Request._callback (/home/travis/build/oaeproject/Hilary/node_modules/oae-rest/lib/util.js:9:8754)
      at Request.self.callback (/home/travis/build/oaeproject/Hilary/node_modules/request/request.js:121:22)
      at Request.EventEmitter.emit (events.js:98:17)
      at Request.<anonymous> (/home/travis/build/oaeproject/Hilary/node_modules/request/request.js:978:14)
      at Request.EventEmitter.emit (events.js:117:20)
      at IncomingMessage.<anonymous> (/home/travis/build/oaeproject/Hilary/node_modules/request/request.js:929:12)
      at IncomingMessage.EventEmitter.emit (events.js:117:20)
      at _stream_readable.js:920:16
```