Update to osmium-export from minjur (both in practice and in the description)

[jenningsanderson]

This depends on next steps from this issue: hotosm/osm-analytics-cruncher#20 (comment), but moving forward, osm-qa-tiles shouldn't be generated with minjur, they should be generated with osmium and the following configuration file: https://github.com/osmlab/osm-qa-tiles/tree/osmium

Once this is resolved, we can more accurately document how they are created, addressing #18 and update the page to link to osmium export instead of minjur.