Make version check command compatible with 2.6+

`redis-server --version` used to look like

    "Redis server version %s (%s:%d)\n"

but antirez/redis@1b247d133351c747a3c6cce8cac64e6830ccab92 changed it to:

    "Redis server v=%s sha=%s:%d malloc=%s\n"

So the install command executes every time.