Fix typo in History.md

'3.0.0 / 2013-12-19' is shown twice.

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* Before:
![2013-12-26 23 39 41](https://f.cloud.github.com/assets/290782/1810902/af4cf1ba-6e3b-11e3-8843-c369cf27e34d.png)

* After:
![2013-12-26 23 40 16](https://f.cloud.github.com/assets/290782/1810903/b2f2db4a-6e3b-11e3-82f0-c54e606dc87a.png)