Add validation to for artist id

Fixes issue #1.

Check if artist id has an existing artist. If it doesn't then the
validation will fail. This prevents the album model saving an album with
an invalid artist id.

- Add validation in Album model to check if artist id references a
  valid artist row.
- Make test suite pass by explicitly setting artist id to 1 in the
  necessary places. The active record fixture docs explain what happen
  when you don't explicitly set an id.
- Add a new test to check that an album cannot be saved without a valid
  artist id.
- Reword testing the test for an empty artist id for readability.
Merge branch 'master' of github.com:jonathanyeong/learn-api