Cleanup xhr event handler

Works around duplicate XHR response issues seen in production.
https://code.google.com/p/chromium/issues/detail?id=159827
https://github.com/madrobby/zepto/commit/8e84f2e52a007b0f1fea740c49d5e669adc3a3dc
Merge pull request #12 from walmartlabs/xhr-cleanup

Cleanup xhr event handler