Old versions of Redis

This fixes the problem described here -> https://github.com/ClearcodeHQ/pytest-dbfixtures/issues/93
In case of using unsupported version of Redis an adequate message will be displayed.

I tested this with Redis 2.4.14 and Redis 2.8.18:
- redis 2.4.14: https://travis-ci.org/marspe/pytest-dbfixtures/builds/43737867
- redis 2.8.18: https://travis-ci.org/marspe/pytest-dbfixtures/builds/43742926

dbfixtures.conf:
- redis 2.4.14: redis_exec: /usr/local/bin/redis-server
- redis 2.8.18: redis_exec: /usr/bin/redis-server
Again, error raised here, would clearly indicate where's the problem. printed message allows to run tests further on, and on some CI you might get access only to tests results, not printed messages.