pg_basebackup streaming: adjust version check msg

Commit d298b50a by Heikki Linnakangas
requested that the version check message be updated at next release, suggesting
that the appropriate text would be “9.3 or later”. The logic used for the check
indicates that the correct text for 9.4 is “9.3 or 9.4”, since the logic would
cause this to fail for later releases.