Check if index exists before adding it. [ci skip]

* I got an error when updating GDK because it already exists.

db/migrate/20160320204112_index_namespaces_on_visibility_level.rb

@@ -1,5 +1,7 @@
 class IndexNamespacesOnVisibilityLevel < ActiveRecord::Migration
   def change
-    add_index :namespaces, :visibility_level
+    unless index_exists?(:namespaces, :visibility_level)
+      add_index :namespaces, :visibility_level
+    end
   end
 end