Revert from codecov back to coveralls

[BrianJKoopman]

Description

This undoes the setup of coverage reporting to codecov.io, back to using coveralls.io.

Motivation and Context

Coverage reports with codecov don't seem to be working properly and I'm not sure why.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.