Add version to .htaccess

Currently if a user does not replace the .htaccess file with the new update this can lead to serious problems in case Apache is used as webserver.

This commit adds the version to the .htaccess file and the update routine fails in case not the newest version is specified in there. This obviously means that every release has to update the version specified in .htaccess as well. But I see no better solution for it.

Fixes https://github.com/owncloud/core/issues/9937 and a lot of other bug reports.

Testplan:
- Update the version in version.php and trigger the update via web => Should fail
- Update the version in version.php, adjust the .htaccess as well and trigger the update vie web => Should work

Known bug that needs to be addressed somewhere else: `occ` does not run the pre-update checks while the web interface does. Something for another PR. Not here.

@karlitschek I know that this is additional pain when creating releases. However, I see no other proper solution for this. Checking for the bad rewrites seems like a very bad solution and will give us trouble in future as well.