Hide concrete5 version number in upgrade page

If the constant APP_VERSION_DISPLAY_IN_HEADER is set to false,
unregistered visitors can still determine the currently installed
version by visiting the page /index.php/tools/required/upgrade.
Let's hide the version number when APP_VERSION_DISPLAY_IN_HEADER is
set to false.
Merge pull request #1064 from mlocati/hide-version-on-upgrade-page

Hide concrete5 version number in upgrade page