Cluster: don't migrate to a master that never had slaves.

Replica migration algorithm modified so that slaves never try to migrate
to masters that were never configured to have slaves in the past.
We want the algorithm to take care of masters that remained without
*working* slaves, but that used to have slaves according to the cluster
configuration.
Cluster test: new unit 05, slave selection.
Example redis.conf: improve slaveof description.
Cluster test: unit 06, slaves with stale data can't failover.
Cluster test: 03 unit speedup, don't send WAIT to slaves.
Cluster test: init test, be patient during restarts.
Cluster test: node reachability condition fixed.
Add Travis CI
Add automated testing for creating clusters

This script creates many different cluster configurations
by varying the number of instances and replica counts
defined at cluster initialization time.  If any cluster
topology fails to converge successfully, the entire test fails.

This script currently works on my OS X system and in a Linux VM.
It's designed to work on a CI server so it just runs
in the background and the system will report if the script
fails.
Add Redis cluster test to CI

The more tests the better!