Fix schema version for background migration spec

spec/lib/gitlab/background_migration/migrate_legacy_artifacts_spec.rb

 require 'spec_helper'
-describe Gitlab::BackgroundMigration::MigrateLegacyArtifacts, :migration, schema: 20180427161409 do
+describe Gitlab::BackgroundMigration::MigrateLegacyArtifacts, :migration, schema: 20180430161409 do
   let(:namespaces) { table(:namespaces) }
   let(:projects) { table(:projects) }
   let(:pipelines) { table(:ci_pipelines) }