Update PLATFORMS.md to reflect new mysql software version

the `2013.03` version of amazon linux now has sport for the 5.5 version of mysql.

```
[root@ip-10-0-2-183 ~]# cat /etc/issue
Amazon Linux AMI release 2013.03
Kernel \r on an \m
[root@ip-10-0-2-183 ~]# yum list | grep mysql-server
mysql-server.noarch                    5.5-1.6.amzn1                amzn-main
```