Decide on what to do about code coverage

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The llvmlite README has a badge pointing to coveralls:

https://github.com/numba/llvmlite/blame/master/README.rst#L11

Which points to: https://coveralls.io/github/numba/llvmlite

Apparently this services was integrated to Travis-CI which is no longer in use for this repository. So, we need to decide if we should reactivate this or drop support.