[ticket/12281] Disable redis on travis until its fixed again

/home/travis/build/phpbb/phpbb/redis/redis.c:6141:34:
error: unknown type name ‘tsrm_ls’
make: *** [redis.lo] Error 1

The command "sh -c "if [ '$TRAVIS_PHP_VERSION' != 'hhvm' ];
then travis/setup-php-extensions.sh; fi"" failed
and exited with 2 during install.

PHPBB3-12281