Give Whitehall more time to finish mappings export

We're currently sometimes downloading the file before it has finished being
generated. This means we only get the file as it stands when we download it.

For UKBA, which is near the end of the file, this meant we weren't getting any
of their mappings.

It currently takes ~42 minutes to generate the file; we were allowing 30 mins.
Let's give it 1hr 15mins.
Merge pull request #163 from alphagov/give_whitehall_export_more_time_to_complete

Give Whitehall more time to finish mappings export