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
@marspe seems like sgn(x) implementation, without usage of if-statement!