Delete releases without project during migration.

platform:  # supported: 2.0.4, 2.1.6, 2.2.3
backlogs:  # supported: 0.9.36
ruby:  # supported: 1.9.3, 1.8.7

Change to sort out this issue: http://forum.redminebacklogs.net/undefined-method-versions-for-nil-NilClass-td4025602.html