Correct if statement so it returns true

The if statement to check for an existing artist would only return
false and never true. This meant that an album could not be saved even
with a valid artist id.

Fix this by adding an else branch that returns true if artist find by id
is not nil (false).
Get all rake tests working

The tests didn't work because the artist id was never 1. Make the test
suite pass by explicitly setting the artist id to 1 in the necessary
places.

The active record fixture website explains what happens when you don't
explicitly set an id:
http://api.rubyonrails.org/v3.2.8/classes/ActiveRecord/Fixtures.html
(see Stable, Autogenerated IDs).

- 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.