Update CurlInterface.php

[phh]

Fix PHPHtmlParser\CurlInterface::get() compatible issue

Getting the following error:

Fatal error: Declaration of PHPHtmlParser\Curl::get(string $url): string must be compatible with PHPHtmlParser\CurlInterface::get($url): string in /home/vagrant/code/vendor/paquettg/php-html-parser/src/PHPHtmlParser/Curl.php on line 11

When using this example:

use PHPHtmlParser\Dom;

$dom = new Dom;
$dom->loadFromUrl('http://google.com');

[phh]

Fixes #163