Load configuration even if libxml entity loader is disabled

This PR addresses a problem which occurs if the bootstrap file contains logic which disables the libxml entity loader.

The following warning will be issued:
> Warning: simplexml_load_file(): I/O warning : failed to load external entity "/home/the/project/path/phpunit.xml" in /home/the/project/path/vendor/brianium
/paratest/src/ParaTest/Runners/PHPUnit/Configuration.php on line 39

And the following exception will be thrown:
>  [RuntimeException]  No path or configuration provided (tests must end with Test.php) 

It would be quite inconvenient to have to bypass the composer autoloading just to handle this case manually. Therefore I just wrapped the ```simplexml_load_file()``` call into a block which enables the libxml entity loading temporarily and restores the setting after the xml file has been parsed.

More information: http://phpsecurity.readthedocs.org/en/latest/Injection-Attacks.html#defenses-against-xml-external-entity-injection