Warning: This build has drifted.
The coverage report for this pull request build may be inaccurate because its base commit is no longer the HEAD of its target branch.
This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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docs: fix coveralls badge not rendering in README

The coveralls native badge serves the SVG via a 302 redirect to an
S3 asset marked Cache-Control: no-cache, which GitHub's Camo image
proxy won't render. Switch to the shields.io coveralls endpoint, which
returns a directly-served, cacheable SVG. The link still points at the
coveralls dashboard.