Update version check file to use json format


![check for updates](https://f.cloud.github.com/assets/214383/2226299/ce2f6974-9a9f-11e3-832d-42d4fd793b42.png)
![check for updates1](https://f.cloud.github.com/assets/214383/2226300/ce38c212-9a9f-11e3-8bb8-ba02024f1fee.png)
![check for updates2](https://f.cloud.github.com/assets/214383/2226301/ce41ffee-9a9f-11e3-99ff-99e1ebafb792.png)


http://tracker.phpbb.com/browse/PHPBB3-9871

Alternative to https://github.com/phpbb/phpbb/pull/2000