Travis-CI failure due to invalid certificate?

The errors appear unrelated to any of my changes.  They start when trying to `load_spitzer_catalog()`, which complains about "certificate verify failed":
https://travis-ci.org/astropy/photutils/jobs/43880947#L1093.

Has anyone seen this error before or have ideas how to fix it?  Or is this simply a transient issue with `travis-ci` (my first suspicion)?  `load_spitzer_catalog()` works fine for me and all tests pass locally.