Tweak Coveralls

[oleibman]

Coveralls recommends using an "official integration" of their product with Github. Also, allow the upload to fail without causing the whole coverage step to fail - there have been some recent problems which they are still working on. I may allow the step to fail once they are done with their changes.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests
• support software

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary