Move command "COMMAND" to the server profile for Redis 2.8.

This command has been backported from the "unstable" branch of Redis
to the "2.8" one, and it is available since 2.8.13.
Switch to "3.0" as the default server profile.

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.