20402 - S4C: Branded Channel Schedule

This PR pertains to [JIRA ticket 20402] (https://jira.dev.bbc.co.uk/browse/IPLAYER-20402) and is related to PR [#3293 in responsive-web] (https://github.com/iplayer/responsive-web/pull/3293)

[SUBMITTED 13/11/2014 @ 17:20]
- Updated `getSlug` in the `Channel` model so that the correct slug is returned for S4C, without breaking existing functionality