Rename unspecified to make it clearer and in line with scrum terminology

'Milestone: None' is very clear, unspecified may mean it has a milestone but it is not clear which one.
This also makes it obvious how to show the backlog from where you can pick issues for the next sprint.

http://feedback.gitlab.com/forums/176466-general/suggestions/3812741-scrum-planning-view-for-issues
http://en.wikipedia.org/wiki/Scrum_(software_development)#Sprint_Backlog