Add double-down job to CI for pull requests

[pshriwise]

I'm hoping to do a release of this package in the near future and word is that double-down is being used with DAGMC more frequently in the wild. As a result, I'll propose that we add it to CI to test PRs into DAGMC. This will avoid reactionary fixes in double-down/DAGMC to broken builds in the develop branch after merging. One example of this could have happened with #714 if I hadn't happened to test it with double-down locally.

I think I originally suggested that we only test double-down on merges to develop to mimic the experimental build functionality of Travis that is unavailable in CI, but I think that was a mistake for the reasons above.

[gonuke]

Resolved by #738