Fix $configFile to handle both files and dirs

Currently, if one runs the following command:

```
/opt/php-cs-fixer> php php-cs-fixer fix crap.php -vvv
```

It would result in search of ...
```
/opt/php-cs-fixer/crap.php/.php_cs
```

... config file which doesn't exist and thus is not loaded. Modifying this results in something like:

```
Loaded config from "/opt/php-cs-fixer/.php_cs"
```