Fix RewriteRule for subdirectory installation

The current rule didn't work under a subdirectory for me (I'm not using Zend, just the .htaccess rules for an app of mine). Fix based on: https://github.com/symfony/symfony-standard/blob/680f09b76dd01355a92a209d1926bf54b7aa50f7/web/.htaccess#L18

Thanks @Tobion and @weierophinney for this gem.