Update wiki for 4.2-5.0, redis version

Created by: cpuguy83

Under the important changes section on the 4.2-5.0 upgrade, please update it to say

• requires redis >= 2.0.0

I understand this is in the troubleshooting section, but this isn't really a troubleshooting type of issue, any user of a previous version of gitlab will likely need to upgrade redis if they are using their distro's repos.