Sentry cleanup

I have now migrated Sentry over to a persistent data store using Amazon RDS, we also decided to start with a fresh install, and therefore have a shiny new DNS name and API key.

This PR:
- Update env variables for dns and api key
- Update Raven.js client to 1.1.16 (needed for setContextTags method)
- Wrap media bootstrap init in its on Raven context to enable tagging and filtering.

Once this is released we can view media specfic errors at: http://frontend.errors.gutools.co.uk/guardian/frontend/?feature=media

I will now send out new Sentry user requests to those whom have asked.

/cc @ironsidevsquincy @mattosborn @rich-nguyen