look for matching legacy_id if id not found

[mlinksva]

Very crude attempt.

See okfn/licenses#37 (comment)

[Floppy]

This seems sensible; as you say it could do with testing. You should just be able to add a test in https://github.com/theodi/odlifier/blob/master/spec/license_spec.rb, like the ones at the end, assuming there is an existing license with a legacy_id you can test? If not, we can fake one with VCR, but have a go with a real one first if available.

cc @olivierthereaux

[mlinksva]

There aren't existing licenses with a legacy id documented, but there should be. I'll follow up after fixing that in okfn/licenses#73

[mlinksva]

Test added.