[openstreetmap/openstreetmap-website] Adjust testing instructions for change from rcov to simplecov (#2066)

[krubokrubo]

The [testing instructions in Contributing.md](https://github.com/openstreetmap/openstreetmap-website/blob/master/CONTRIBUTING.md) say you can generate test coverage stats with: `sudo gem install rcov; rcov -x gems test/*/*.rb`

However, [rcov does not work with Ruby 1.9+](https://github.com/relevance/rcov/issues/99) and says to use simplecov instead.  Ideally, Contributing.md should be updated with the best practice for how to do so.  I figured out [one way to do it](https://github.com/lemurheavy/coveralls-public/issues/881) but I'm not sure if it's the best practice.

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