Fix ERFA import error

[weaverba137]

This PR fixes #134. @sbailey, could you please test in the candidate desiconda environment?

[sbailey]

Thanks. I confirm that:

1. this works with astropy 4.0.1.post1 used in 20200801-1.4.0-spec (current default, i.e. it doesn't break backwards compatibility)
2. this works with astropy 4.2 in draft desiconda release 20210326-1.5.0-spec (good)
3. current master does not work with 20210326-1.5.0-spec (i.e. I'm testing what I think I'm testing in (2))

i.e. ok to merge.

That being said, unit tests are now failing under astropy 4.2 for a different reason for which I will open a different ticket to keep PR topics separate.