Update .travis.yml

Author: k1LoW

.travis.yml

@@ -8,7 +8,7 @@ php:
 env:
   - DB=mysql CAKEPHP=2.3.10
   - DB=mysql CAKEPHP=2.4.10
-  - DB=mysql CAKEPHP=2.5.5
+  - DB=mysql CAKEPHP=2.5.6
 
 before_script:
   - composer install --dev