Using ::class instead of type alias (working w/ sf3, see #148)

README.md

@@ -99,8 +99,9 @@ You can use the "captcha" type in your forms this way:
 
 ```php
 <?php
+    use Gregwar\CaptchaBundle\Type\CaptchaType;
     // ...
-    $builder->add('captcha', 'captcha'); // That's all !
+    $builder->add('captcha', CaptchaType::class); // That's all !
     // ...
 ```
 
@@ -147,8 +148,9 @@ Example :
 
 ```php
 <?php
+    use Gregwar\CaptchaBundle\Type\CaptchaType;
     // ...
-    $builder->add('captcha', 'captcha', array(
+    $builder->add('captcha', CaptchaType::class, array(
         'width' => 200,
         'height' => 50,
         'length' => 6,

Type/CaptchaType.php

@@ -11,6 +11,7 @@ use Symfony\Component\OptionsResolver\OptionsResolver;
 use Symfony\Component\OptionsResolver\OptionsResolverInterface;
 use Symfony\Component\Form\FormEvents;
 use Symfony\Component\Translation\TranslatorInterface;
+use Symfony\Component\Form\Extension\Core\Type\TextType;
 
 use Gregwar\CaptchaBundle\Validator\CaptchaValidator;
 use Gregwar\CaptchaBundle\Generator\CaptchaGenerator;
@@ -143,7 +144,7 @@ class CaptchaType extends AbstractType
      */
     public function getParent()
     {
-        return 'text';
+        return TextType::class;
     }
 
     /**