Fix server md5 check

At some point (when?) the minecraft_server.1.6.4.jar must have changed causing installation of FML to fail.

I first noticed this issue when ProjectRed's Travis-CI failed ([link](https://travis-ci.org/MrTJP/ProjectRed/jobs/22063714)).
I know 1.6.4 is no longer the focus but for anyone wishing to use it this will fix the problem.