Switch to FTP for Travis CI fetch of "jup310.bsp"
Switch to FTP for Travis CI fetch of "jup310.bsp"

Trying to grab it with curl using HTTPS was giving the error:
```
url: (35) error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure
```

skyfielders/python-skyfield,skyfielders/python-skyfield