Update 20130419190306_allow_merges_for_forks.rb

Postgress db migrate fails.(5.4=>6.0)

db/migrate/20130419190306_allow_merges_for_forks.rb

 class AllowMergesForForks < ActiveRecord::Migration
   def self.up
-    add_column :merge_requests, :target_project_id, :integer, :null => false
+    add_column :merge_requests, :target_project_id, :integer, :null => true
     MergeRequest.update_all("target_project_id = project_id")
+    change_column :merge_requests, :target_project_id, :integer, :null => false
     rename_column :merge_requests, :project_id, :source_project_id
   end