Use DIRECTORY_SEPARATOR instead of / in test. Fixes codeguy/Slim/#896.

It looks like tests always fail in Windows. Failing line in [setTemplatesDirectory()](https://github.com/codeguy/Slim/blob/master/Slim/View.php#L207) is the following:

```php
$this->templatesDirectory = rtrim($directory, DIRECTORY_SEPARATOR);
````
[ViewTest::testSetTemplatesDirectory()](https://github.com/codeguy/Slim/blob/master/tests/ViewTest.php#L165) has hardcoded "/" character which causes test to fail in Windows. Slim code itself is correct. Test is not portable.

```php
$view->setTemplatesDirectory('templates/');
```

PR has not been tested with actual Windows machine. PR is against master because this test does not exist in development anymore.