Replace is_callable checks with type hints

Also removes tests checking the exceptions thrown from
the removed is_callable checks.

Author: mpajunen

PluralizationRules.php

@@ -189,12 +189,10 @@ public static function get($number, $locale)
     /**
      * Overrides the default plural rule for a given locale.
      *
-     * @param string $rule   A PHP callable
-     * @param string $locale The locale
-     *
-     * @throws \LogicException
+     * @param callable $rule   A PHP callable
+     * @param string   $locale The locale
      */
-    public static function set($rule, $locale)
+    public static function set(callable $rule, $locale)
     {
         if ('pt_BR' === $locale) {
             // temporary set a locale for brazilian
@@ -205,10 +203,6 @@ public static function set($rule, $locale)
             $locale = substr($locale, 0, -strlen(strrchr($locale, '_')));
         }
 
-        if (!is_callable($rule)) {
-            throw new \LogicException('The given rule can not be called');
-        }
-
         self::$rules[$locale] = $rule;
     }
 }