Fix syntax error related to throwing a function

When submitting a call to the API and receiving unparseable JSON, I get the following message:

```Function name must be a string in /path/to/factual/factual-php-driver/FactualResponse.php on line 28```

This is a result of trying to throw an exception casted as a function.
Merge pull request #25 from trascopur/patch-1

Fix syntax error related to throwing a function