Redis test should be ignored if connecting isn't possible.

1) phpbb_cache_redis_driver_test::test_get_put_exists
Could not connect to redis server
/home/travis/build/phpbb/phpbb/phpBB/phpbb/cache/driver/redis.php:67
/home/travis/build/phpbb/phpbb/tests/cache/redis_driver_test.php:46
2) phpbb_cache_redis_driver_test::test_purge
Could not connect to redis server
/home/travis/build/phpbb/phpbb/phpBB/phpbb/cache/driver/redis.php:67
/home/travis/build/phpbb/phpbb/tests/cache/redis_driver_test.php:46
3) phpbb_cache_redis_driver_test::test_destroy
Could not connect to redis server
/home/travis/build/phpbb/phpbb/phpBB/phpbb/cache/driver/redis.php:67
/home/travis/build/phpbb/phpbb/tests/cache/redis_driver_test.php:46
4) phpbb_cache_redis_driver_test::test_cache_sql
Could not connect to redis server
/home/travis/build/phpbb/phpbb/phpBB/phpbb/cache/driver/redis.php:67
/home/travis/build/phpbb/phpbb/tests/cache/redis_driver_test.php:46