Support multi-licensed dependencies

When a dependency has multiple licenses, it's approved if any one of its licenses is whitelisted.

In this PR we modify `LicenseFinder::Dependency` to store an array of licenses. This array is serialized into a JSON string and stored in the DB.

This PR includes the cleanup in #103.
Good catch. I made this change in my first pass converting licenses to an array, but forgot to update it when a dependency could have more than one license. Update coming up.