Fix extra element bug with `$_SERVER['argc']` and `$_SERVER['argv']`.

This fixes #688.

When a file is executed without the `--file` argument, `$_SERVER['argv']`
contains an extraneous empty string at the beginning of the array.

Consider the following:

__test.php:__

```php

<?php
 
print_r($argv);
print_r($_SERVER['argv']);

```

__hhvm --file:__

``` 
whatthejeff@ubuntu-hhvm:~$ hhvm --file test.php
Array
(
    [0] => test.php
)
Array
(
    [0] => test.php
)
```

__hhvm:__

```
whatthejeff@ubuntu-hhvm:~$ hhvm test.php
Array
(
    [0] => test.php
)
Array
(
    [0] =>
    [1] => test.php
)
```