Fix archive without phar extension - #1127

Fix [#1127](http://www.phing.info/trac/ticket/1127)

When using the phar archive without the phar extension, the following error occurs: 

```
PHP Warning:  include(phar:///usr/local/bin/phing/bin/phing.php): failed to open stream: phar error: invalid url or non-existent phar "phar:///usr/local/bin/phing/bin/phing.php" in /usr/local/bin/phing on line 11
```

This fixes the error by using a relative include path in the phar-stub