fix flush to use Enterprise_PageCache_Model_Cache only for 1.11 and up, ...

...rather than 1.10.  Fixes "Fatal error: Class 'Enterprise_PageCache_Model_Cache' not found in phar:///usr/local/bin/mr/src/N98/Magento/Command/Cache/FlushCommand.php on line 43" when running n98-magerun cache:flush on 1.10.0.1.  The Cache model wasn't added until 1.11