Bugfix release 0.6.4

The fix in Eyegrade 0.6.3 for issue #108 (the accept button remains sometimes disabled in the edit student id dialog) didn't actually fix the problem in some situations. I've released Eyegrade 0.6.4 with a fix that I hope finally solves the problem.

As always, you can update with git pull if you installed Eyegrade through git.