support astropy 4.2; switch to github actions for testing

[sbailey]

This PR

1. fixes reading horizons_2020_week1_moon.csv broken under astropy 4.2 #137 by fixing the parsing of horizons moon ephem under astropy 4.2
2. fixes revive tests with GitHub actions #133 by switching from travis to github actions for CI testing

I suspect that (2) will take me a few more iterations to get the config right, so apologies in advance for the spam while I test the tests.

[sbailey]

@weaverba137 could you setup a COVERALLS_REPO_TOKEN for desisurvey? I'm not sure if we ever had coveralls configured for desisurvey, and/or what the procedure is to do that. Thanks.

[weaverba137]

Done!

[sbailey]

Thanks @weaverba137 .

@dkirkby @schlafly I think this is a straightforward PR to replace travis tests with github actions, and to fix tests under astropy 4.2 . It does not change any core code (only tests). I'll leave this open for a bit if you have any comments or requests, but otherwise I plan to merge this by the end of today and make a new desisurvey tag.

[schlafly]

Let me just thank you for fixing this. I don't see any issues. Feel free to merge.

[dkirkby]

Yes, thanks @weaverba137!