Redis 3.0 does not really change much in terms of commands (most of
the ones implemented in the "unstable" branch have been backported to
the "2.8" branch after all) aside from a few cluster-related ones, so
we can bump the default version without worries.
We now have a base test case class for Predis (namely PredisTestCase)
grouping various commonly used utility methods shared by all of the
tests in the suite, greatly improving reusability.