Renames AddMissingColumnsToProjectMirrorData to EnsureMissingColumnsToProjectMirrorData

db/migrate/20180503175053_add_missing_columns_to_project_mirror_data.rb → db/migrate/20180503175053_ensure_missing_columns_to_project_mirror_data.rb

-class AddMissingColumnsToProjectMirrorData < ActiveRecord::Migration
+class EnsureMissingColumnsToProjectMirrorData < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false