Prevent calls to GitLab.com during automated tests

Right now, we have to provide a production ACCESS_TOKEN environment variable that is valid on GitLab.com for the tests to work. This is a great first iteration but causes obstacles for future development:

• Local tests depend on having a valid GitLab.com access token (undocumented)
• We can not test for errors during network requests
• If we want to add features like #53, we have to mock GitLab responses because we could cause accidental damage to data stored in the user’s account