Renamed INSTALL file

During the installation the installer checks for the URL `/install/test_url/20/20/`.

Now if you have `URL_REWRITING_ALL` enabled, created a proper `.htaccess` and you're on a case-insensitve filesystem (e.g. my development Mac), Apache will match the `INSTALL` and you're not able to install concrete5 because it fails at the check «Supports concrete5 request URLs».

Found quite a few forum posts about similar problems. e.g. http://www.concrete5.org/community/forums/installation/support-for-c5-request-urls1/