Add default value to repo_id field

Prior to this commit, the field `repo_id` in `pull_requests` had no
default value. this caused ActiveRecord to complain about "Cannot
add a NOT NULL column with default value NULL" when running migrations
during rake tests. This commit adds a default value of 0 to the repo_id
field.

Note that: 1) The rake task job:import should always provide a repo_id,
and thus the default should never end up being used in production, and
2) there should not be a repo with id 0 anyway.
Remove obsolete generate_repositories method

Prior to this commit, the `generate_repositories` method was still
floating around in the application class, even though that method is now
unesecary now that repository data is stored in its own table. This
commit removes that method.
Remove gigantic block of commented code
Remove whitespace
Rewrite spec tests for new database format

Prior to this commit, the spec tests had not been updated to reflect the
the new database structure, particularly the fact that the Repository
class was a model for a table now. This commit updates the tests to
reflect that.