Add deprecation warning to cache package.

There was only one in Cache, and none in Log :)

src/Cache/Cache.php

@@ -143,6 +143,7 @@ class Cache
      */
     public static function registry(ObjectRegistry $registry = null)
     {
+        deprecationWarning('Use Cache::getRegistry() and Cache::setRegistry() instead.');
         if ($registry) {
             static::setRegistry($registry);
         }

tests/TestCase/Cache/CacheTest.php

@@ -813,6 +813,7 @@ class CacheTest extends TestCase
     /**
      * test registry method
      *
+     * @group deprecated
      * @return void
      */
     public function testRegistry()
@@ -823,6 +824,7 @@ class CacheTest extends TestCase
     /**
      * test registry method setting
      *
+     * @group deprecated
      * @return void
      */
     public function testRegistrySet()