add PHP 5.4, 5.6 to .travis.yml

@sasezaki 

As @GeeH pointed out in https://github.com/zendframework/modules.zendframework.com/pull/234#issuecomment-65405124 and in a conversation we had on IRC, the server on which the app is running currently has PHP 5.4 installed, so I think we shouldn't allow any failures in regard to PHP 5.4.