Use new coverage url in README

README.md

-Thalia Website [![coverage report](https://gitlab.science.ru.nl/thalia/concrexit/badges/master/coverage.svg)](https://coverage.technicie.nl/master)
+Thalia Website [![coverage report](https://gitlab.science.ru.nl/thalia/concrexit/badges/master/coverage.svg)](https://thalia-coverage.s3.amazonaws.com/master/index.html)
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 New new Thalia website, now with extra Django.