Switch PROD to use new ajax url.

Reason:

This will enable us to move totally away from m.guardian.co.uk (we will be able to redirect all m. traffic to www.)

Success looks like:

All ajax endpoints work on the new URL.
Merge branch 'master' of github.com:guardian/frontend into dotcom
Merge pull request #1123 from guardian/dotcom

Switch PROD to use new ajax url.