Don't use cached informations for app version

When installing an app from the appstore the `\OC_App::getAppVersion` code is triggered twice:

- First when the downloader tries to compare the current version to the new version on the appstore to check if there is a newer version. This protects against downgrade attacks and is implemented in `\OC\Installer::downloadApp`.
- Second, when the app is actually installed the current version is written to the database. (`\OC\Installer::installApp`)

This fails however when the version is actually cached. Because in step 1 the cached version will be set to "0" and then be reused in the second step.

While this is probably not the cleanest version I assume this is an approach that is least invasive. Feedback and suggestions welcome :)

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>