Fix for random unloving/loving of tracks on last.fm

[chrismou]

See #390

Essentially, the rating observer was being called before the track info was updated, meaning that for every song it was applying the loved status of the previous song. This PR should fix that.

I've logged a separate issue at #425 to look into stopping the multiple unnecessarily api calls at the start of every song.

[jacobwgillespie]

LGTM (though separately we should probably change the CHANGELOG on each PR, so you may want to add an entry)

[chrismou]

Ah OK, so we're adding our own changelog entries? No worries, I'll update it before I merge.

[chrismou]

Interesting point though - if we're referencing PRs from the changelog (which is a good idea), this means we have to PR to get the PR url, then update the changelog, rebase, and push an updated PR straight away. Which isn't the end of the world, but it is a bit of a PITA. It also means we have to ask contributers to update the changelog.

tbh, I wouldn't have an issue with the merger pushing a changelog change directly to master once it's been merged, as it's not a functional change. As long as it's just a change to that file, it shouldn't be an issue and it'll only ever be one of us that does it.