Use correct collection lookup for graph periods

collection.options.period is always undefined, so this would return "week" irrespective of the actual configured period.

Update cheapseats pointer to include tests that this caused to fail.
Merge pull request #672 from alphagov/bugfix/single-timeseries-durations

Use correct collection lookup for graph periods
Correct description text on Prison Visits dashboard
Merge pull request #674 from alphagov/chore/text-change-on-prison-visits-dashboard

Correct description text on Prison Visits dashboard