Add version-check for extension

Hi,

please add a version-check to the extension so an admin is able to check for new versions within the ACP of phpBB.

An example how this can be achieved is shown here:

https://github.com/s9e/phpbb-ext-mediaembed/blob/master/composer.json#L18-L22
https://github.com/s9e/phpbb-ext-mediaembed/blob/master/versions.json

PR in #69