fix for PHP warning "is_file(): open_basedir restriction in effect", round 2

This reverts commit d234b8e2128356a5d7c76e7d7048c21a26bd49c2 in PR #47 and returns back the is_dir() check.

I was working with too many hosts simultaneously and checked the file_exists() method on a host that worked perfectly even before the fix and as I updated the files on the problematic host the problems returned and the warning message changed to:
`````plain
PHP Warning: file_exists(): open_basedir restriction in effect. File(/home/users/.../web/user/plugins/.gitkeep/.gitkeep.yaml) is not within the allowed path(s): (/home/users/.../:/usr/share/pear/) in .../web/system/src/Grav/Common/Config.php:237
````
I'm sorry for this and have triple-checked that this code really finally fixes the issue.