Rescue from Redis::BaseError in #ab_test

Redis defines a number of different exception classes (see
https://github.com/redis/redis-rb/blob/1bd43346cdf71ba69176612714616289a5a70ed4/lib/redis/errors.rb
for details). When Redis raises an exception, we need to be able to have
the db_failover configuration used, whether it's a failure to connect, a
timeout, or an invalid command.

We ran into this issue when our Redis instance ran out of memory. In
that case (and most others), Split should be able to handle things
gracefully.

lib/split/helper.rb

@@ -16,7 +16,7 @@ def ab_test(metric_descriptor, control = nil, *alternatives)
         else
           control_variable(experiment.control)
         end
-      rescue Errno::ECONNREFUSED, Redis::CannotConnectError, SocketError => e
+      rescue Errno::ECONNREFUSED, Redis::BaseError, SocketError => e
         raise(e) unless Split.configuration.db_failover
         Split.configuration.db_failover_on_db_error.call(e)