Fix version retrieval for TYPO3 6.0

There is no `cache_extensions` table in 6.0 anymore, so querying
this table for the extension version will fail in every case.

For TYPO3 6.0, the `ExtensionRepository` is used to query the version.
Merge pull request #7 from githubnemo/master

Fix version retrieval for TYPO3 6.0