Set travel-advice index cache time to 30 mins

Now that the "recently updated" section has been removed from the index
html view, there's no need for a short cache time.  This sets it back to
the default (30 mins).

Note: the atom view still updates frequently, so needs the 5 minute
cache time.
Merge pull request #489 from alphagov/improve_travel_advice_index_cacheability

Set travel-advice index cache time to 30 mins