#576 bump up required MySQL version

[XimeX]

I also think we should push the min MySQL version up to 5.1 or 5.5.
Maybe only to 5.1 but show an info to the user that 5.1 didn't get any more updates and would be insecure. (https://en.wikipedia.org/wiki/MySQL#Versions)
Maybe we should also list the required MariaDB version (fork from MySQL)

I think the main reason that there are so many old PHP & MySQL versions is debian 6. but debian 6 only gets updates to Sept 2015. My hosting provider is planning to move to debian 8 in the next months.

Security is for me one of the most important things ;-)