Update to SOFA version 13

[mhvk]

As noted by @rmathar (#44 (comment)), SOFA version 13 is out, and we should update ERFA to it, since it solves #40 and #41. It is also a good opportunity to finally get in #42...

[bsipocz]

@mhvk @eteq - Is there any chance to get a new erfa version out to bundle with astropy v2.0?

[mhvk]

We should -- but I fear I won't be able to do it...

[eteq]

@bsipocz @mhvk - we could pull it into master today (and pull it into Astropy), but I don't think I can do the full release. So it depends on whether we want a "pre-released" version in Astropy. (We'd still have 2 weeks to iron out any changes, since those could be post feature-freeze but pre-Astropy release). I don't see that as a new "feature".

@astrofrog or @taldcroft might want to weigh in too, given that it's an Astropy procedural question

[bsipocz]

I feel that your proposal is fine for feature freeze, given if you can do a full release before we have final v2.0. (Updating externals doesn't seem to be a new feature to me, but erfa is critical enough that we need the RC testing for it I think.)

[mhvk]

Agreed with @bsipocz - pre-release should be fine. I think it would be very good to have even just the erfa version exposed in astropy 2.0.