Fall back when value is #N/A

[kayue]

getCalculatedValue() doesn't work properly with my Excel. I have created a test script below, it is returnning '#N/A' but it shouldn't.

File: http://ge.tt/8jaJfiM2/v/0

#!/usr/bin/env php
<?php
(@include_once __DIR__ . '/../vendor/autoload.php') || @include_once __DIR__ . '/../../../autoload.php';
dump((new PHPExcel_Reader_Excel2007())
    ->setReadDataOnly(true)
    ->load($argv[1])
    ->getActiveSheet()
    ->getCell('AL20')
    ->getCalculatedValue()
);

This PR should fix it, kindly review.

[kayue]

ping @MarkBaker

[kayue]

ping @MarkBaker...

[kayue]

@MarkBaker any feedback?

[kayue]

ping @MarkBaker...

[kayue]

ping @MarkBaker