Migrate from CodeCov to Coveralls

[MikeMcQuaid]

This is the last thing blocking our migration from Travis CI to Azure Pipelines.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[reitermarkus]

This is the last thing blocking our migration from Travis CI to Azure Pipelines.

Why doesn't CodeCov work on Azure?

[MikeMcQuaid]

Why doesn't CodeCov work on Azure?

Despite the merge of codecov/codecov-ruby#38 their server still actively rejects anything from Azure. I've been trying to get something out of their support for months to no avail. It's the last thing mandating the use of Travis CI so I want to get it over and done with.