Ensure CI builds for branches in 3rd party repositories build successfully

[lucaswoj]

ref #2453

[mourner]

I know remember that there was a known limitation to PR branches on Circle, but @jfirebaugh should know more.

[lucaswoj]

This looks like Coverall's fault. If there isn't an easy solution, I think we should ignore errors from the coveralls CLI.

[jfirebaugh]

lemurheavy/coveralls-public#632 and lemurheavy/coveralls-public#487 suggest some troubleshooting steps.

[lucaswoj]

Looks like we need to set COVERALLS_REPO_TOKEN in our CI environment. I have mitigated the problem temporarily by preventing a coveralls error from causing the build to fail.

[lucaswoj]

I researched this a little further and found that CircleCI intentionally omits environment variables from forked builds. It even says so in the build log:

The only way we could enable coveralls for forked builds is to make our coveralls repo token public.

As a compromise, I have removed "coveralls" from the list of required checks.

Long term: I wonder if we can hook the coverage stats into our benchmarking system #1362 to get around this design flaw in coveralls using a similar approach as #3495.