Let integer be included in slug

lib/Gedmo/Sluggable/Mapping/Driver/Annotation.php

@@ -46,7 +46,8 @@ class Annotation implements AnnotationDriverInterface
      */
     private $validTypes = array(
         'string',
-        'text'
+        'text',
+        'integer'
     );
 
     /**

lib/Gedmo/Sluggable/Mapping/Driver/Xml.php

@@ -29,7 +29,8 @@ class Xml extends BaseXml
      */
     private $validTypes = array(
         'string',
-        'text'
+        'text',
+        'integer'
     );
 
     /**

lib/Gedmo/Sluggable/Mapping/Driver/Yaml.php

@@ -34,7 +34,8 @@ class Yaml extends File implements Driver
      */
     private $validTypes = array(
         'string',
-        'text'
+        'text',
+        'integer',
     );
 
     /**