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

Example:
PHP Warning: is_file(): open_basedir restriction in effect. File(/home/users/.../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
reverted back to file-only checks, changed to file_exists()
Merge pull request #47 from rindeal/feature/config-fixes

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