[Form] Fixed wrong Inspection for FormTypeExtensions #483
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FormTypeExtensions must implement
\Symfony\Component\Form\FormTypeExtensionInterfacenot\Symfony\Component\Form\FormExtensionInterface. Symfony's abstract classAbstractTypeExtensiondoes this aswell.See http://symfony.com/doc/current/reference/dic_tags.html#form-type-extension
and http://symfony.com/doc/current/cookbook/form/create_form_type_extension.html