give peridot specs dir and use proper exit code for cli failures

travis was passing because peridot was exiting 0 because of `die()` when there was no spec dir argument
@brianium, can you also do this to this part:

```php
if (!$autoload) {
    die(
        'You need to set up the project dependencies using the following commands:' . PHP_EOL .
        'curl -s http://getcomposer.org/installer | php' . PHP_EOL .
        'php composer.phar install' . PHP_EOL
    );
}
```