You can't revert back the version of your Minecraft Pocket Edition.
You can try to get an old version's LEGIT .ipa file and installing by iTunes...
If you are unable to do that, all you can do is to look for servers with newer versions or contact the owner of that particular server and ask him/her to update the server.
The developer of that particular server you stated in the question before I edited it out responded that it is a big issue that people who are fan of that server are switching to other servers and this is really important to them.
Then he clarified it here that they will be updating very soon.
If you are playing on other servers too, be sure to contact admins/moderators/developers/insert title here of that server and ask them to update their servers.