Keep list of Bank Holidays up to date

[joshpencheon]

The WorkingDays concerns tracks public holidays (for England/Wales), but needs manual updating. As a result, it has fallen stale.

We should make use of https://www.gov.uk/bank-holidays/england-and-wales.json, perhaps baking the cache at rake build time. We can't presume to have internet connectivity once ndr_support is being used.

[timgentry]

@joshpencheon regardless of the solution, we could add a test that goes and gets https://www.gov.uk/bank-holidays/england-and-wales.json and assert that all the returned bank holidays are considered holidays.

[timgentry]

If we did use the holidays gem I think we would use the :gb_eaw region