Use updated version of simple-html-dom-parser library

Replaces an old version with one updated for PHP 7.3. This is needed b/c
PHP 7.3 changes how it handles regular expressions; the PCRE extension was
upgraded: https://secure.php.net/manual/en/migration73.other-changes.php#migration73.other-changes.pcre

Closes #173

composer.json

@@ -3,7 +3,9 @@
     "description": "ConvertKit WordPress plugin",
     "type": "project",
     "license": "GPLv3",
-    "require": {},
+    "require": {
+        "kub-at/php-simple-html-dom-parser": "^1.7"
+    },
     "require-dev": {
         "phpunit/phpunit": "^6.5",
         "vlucas/phpdotenv": "^2.4",

includes/class-convertkit-api.php

@@ -1,6 +1,6 @@
 <?php
 /**
- * ConvertKit PI class
+ * ConvertKit API class
  *
  * @package ConvertKit
  * @author ConvertKit
@@ -396,10 +396,6 @@ public function get_resource( $url ) {
 
 			if ( ! is_wp_error( $response ) ) {
 
-				if ( ! function_exists( 'str_get_html' ) ) {
-					require_once( CONVERTKIT_PLUGIN_PATH . '/lib/simple-html-dom/simple-html-dom.php' );
-				}
-
 				if ( ! function_exists( 'url_to_absolute' ) ) {
 					require_once( CONVERTKIT_PLUGIN_PATH . '/lib/url-to-absolute/url-to-absolute.php' );
 				}
@@ -413,7 +409,8 @@ public function get_resource( $url ) {
 
 				$body = wp_remote_retrieve_body( $response );
 
-				$html = str_get_html( $body );
+				/** @var \simple_html_dom\simple_html_dom $html */
+				$html = \KubAT\PhpSimple\HtmlDomParser::str_get_html( $body );
 				foreach ( $html->find( 'a, link' ) as $element ) {
 					if ( isset( $element->href ) ) {
 						$element->href = url_to_absolute( $url, $element->href );