[rollbar] Pull Request #23: Update rollbar.js snippet to version 2.5.3

_Migrated from [#23](https://github.com/segment-integrations/analytics.js-integration-rollbar/pull/23) by @waltjones_

Updates the rollbar.js snippet to the latest version, from: https://github.com/rollbar/rollbar.js/blob/v2.5.3/dist/rollbar.snippet.js