Fix data resource locations

[tomviner]

I found I could not use Maven to collect data from the electoral commission website because 2 URLs no longer worked.

1. The uk_2010_results data is no longer on the electoral commission website due to their policy:

We publish information about the most recent election or referendum, and the one before it. For example, you'll find information about the UK general elections that took place in 2017 and in 2015.

From https://www.electoralcommission.org.uk/who-we-are-and-what-we-do/elections-and-referendums/past-elections-and-referendums

2. The uk_2015_results URL has changed.

[john-sandall]

On the plus side, they've had a major site redesign and the data they do have looks a lot easier to find. On the negative side, why would they take down such fundamental and important data!

I'm not especially comfortable using the S3-hosted backup here as primary data source, but it'll have to do until we can find a new primary.

Thanks for doing this.