Also, hardcode expected values instead of using helper functions strptime_utc() and unstrip(), per https://testing.googleblog.com/2014/07/testing-on-toilet-dont-put-logic-in.html (Don't Put Logic in Tests).

This is an example of a FITS image with BJD_TDB in the header.

Contributed by David Valls-Gabaud.

At the same time, read_barycentric_date() becomes private. In this manner, FITSImage.date() is the single entry point, branching to either _read_uncorrected_date() (for the regular code path) or _read_barycentric_date() to read Baycentric Julian Dates. This allows us to extend the existing interface to add support for the BJD_TDB FITS keyword without having to update the rest of the pipeline.

…iables

HH MM SS.ss and DD MM SS.ss are not standard, but common enough to merit support.

Don't talk to production in tests.

… are accepted

https://github.com/tronsgaard/barycorr/pulls

Travis CI clones Git submodules by default:
https://docs.travis-ci.com/user/customizing-the-build#git-submodules

This fixes the "Permission denied (publickey)" error:

```
Cloning into '/home/travis/build/vterron/lemon/barycorr'...
Warning: Permanently added the RSA host key for IP address '140.82.112.3' to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.
```

https://stackoverflow.com/q/15674064/184363

Otherwise new users won't see the new features until v4 is released.

And propagate them to _read_barycentric_date().