Stub Issue#closed? and Issue#commenting_users

Stub there methods in test to avoid getting error:

`GitHubBub::RequestError: message: 'API rate limit exceeded for my.ip.add.res.'`
Remove usernames from database.yml
Update schema
Merge pull request #180 from mrgilman/remove_db_usernames

Remove usernames from database.yml
Merge pull request #181 from mrgilman/update_schema

Update schema
Merge pull request #177 from ck3g/fix_issue_valid_for_user_test

Stub Issue#closed? and Issue#commenting_users