Change fixture default host to localhost

[asdaraujo]

@dpkp, I think the current errors with the Travis build may be because the VM have IPv6 disabled and since the fixtures are forcing the use of IPv6 the Kafka broker cannot bind to a valid address. If this is indeed the case, however, I'm not sure what happened recently that caused builds start failing.

I had a look at the KafkaFixture code and saw the comment saying that if the broker binds to an IPv4 address, then "kafka-python will attempt to connect on ::1 and fail".
I'm not sure why that's the case. I checked the code and couldn't find an explanation. Maybe this has been changed since?

I'm also not quite sure what's the use of the bracket notation for the broker address (e.g. [::1]). I couldn't find a reference to it in the Kafka documentation. I noticed that it doesn't work with "localhost". If I set the default address to [localhost] the tests fail. However, if the address is set to simply localhost it seems to work fine on hosts with IPv6 enabled or disabled.

I tested this change on two different VMs: one with IPv6 enabled and the other with IPv6 disabled. All tests passed. So I'm sending this PR to see how it goes on Travis.

[dpkp]

Thanks for the fix! I don't recall the context behind the fixture change to require IPv6. This works on travis in all test configurations and I verified it works for me locally as well. Awesome!