Support Ruby 2.1 and better explain the memory requirements.
Merge branch 'ruby2.1' into 'master'

Ruby2.1

Fixed #1075
Merge request drag-n-drop on milestone page

Allows moving milestone between 3 columns: unassigned, assigned,
declined

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'milestone-mr-dragging' into 'master'

Milestone merge request dragging

This is part of #1207.