Fix for PHP 5.3 support #8

I'm aware the plugin has only been tested on 5.5, and developing locally with 5.4 works a treat. Our development server runs 5.3.29, and runs across an error in fieldtypes/ButtonBox_TextSizeFieldType.php and fieldtypes/ButtonBox_ColoursFieldType.php

Basically, It just doesn't like the direct calling method when referencing an array index (why this was never supported a long time ago is beyond me).

Up to you if you wish to add official support for this, and in case anyone else needs a fix for this issue:

protected function getDefaultValue()
{
    foreach ($this->getOptions() as $option)
    {
        if ( !empty($option['default']) )
        {
            return $option['value'];
        }
    }
    return $this->getOptions()[0]['value'];
}

protected function getDefaultValue()
{
    $options = $this->getOptions();

    foreach ($options as $option)
    {
        if ( !empty($option['default']) )
        {
            return $option['value'];
        }
    }
    return $options[0]['value'];
}