Introduce testing using 'goutte' as optional clearing mode

There is no more assumption about possible use of APCu.

For PHP 5.5 and above, I would suggest in a separate PR https://github.com/thierrymarianne/ApcBundle/commit/8404f711867537d61334d0ba0de7579fc0f228d9 :
- to check if APCu extension is loaded before trying to clear user cache  
- to throw an exception when simply trying to use the command for main opcode cache clearing