Merge branch 'lock_fix' into 'master'

Remove default value for lock_version

## What does this MR do?

From the very beginning we added `lock_version` column with default value, then we reverted that MR. A bit later we added that migration again but column's default value has been removed from migration. So if you installed GitLab form master it's possible that you have default value, which caused  the bug.

We don't need to change a CHANGELOG here I think.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21527 and https://dev.gitlab.org/gitlab/organization/issues/971

See merge request !6111

db/migrate/20160827011312_ensure_lock_version_has_no_default.rb

+class EnsureLockVersionHasNoDefault < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    change_column_default :issues, :lock_version, nil
+    change_column_default :merge_requests, :lock_version, nil
+
+    execute('UPDATE issues SET lock_version = 1 WHERE lock_version = 0')
+    execute('UPDATE merge_requests SET lock_version = 1 WHERE lock_version = 0')
+  end
+
+  def down
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160824103857) do
+ActiveRecord::Schema.define(version: 20160827011312) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"