Fix SL_swap class issue

Move SL_swap to the main class attribute for this element, as the duplicate class attribute was being removed

@kylerush Not sure if this will fix it but the SL_swap class is currently not working on staging (see: http://smartling-staging.s3-website-us-east-1.amazonaws.com/customers/customer-stories/ and http://staging.optimizely.de/customers/customer-stories/). Looks like Chrome removes it, and Smartling is not picking it up as well.