fetch login or password from env

@pengwynn 

my machine had BOXEN_GITHUB_LOGIN set but not the password,
so the old code never asked for my password, this fixes it by asking for whatever is unknown

error I got prior looked like this:
```
--> Oh, looks like you've provided your username and password as environmental variables...

/opt/boxen/repo/.bundle/ruby/2.0.0/gems/octokit-2.3.1/lib/octokit/response/raise_error.rb:16:in `on_complete': GET https://api.github.com/authorizations?&per_page=100: 404 - Not Found // See: http://developer.github.com/v3 (Octokit::NotFound)
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/faraday-0.8.8/lib/faraday/response.rb:9:in `block in call'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/faraday-0.8.8/lib/faraday/response.rb:63:in `on_complete'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/faraday-0.8.8/lib/faraday/response.rb:8:in `call'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/faraday-0.8.8/lib/faraday/connection.rb:253:in `run_request'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/faraday-0.8.8/lib/faraday/connection.rb:106:in `get'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/sawyer-0.5.1/lib/sawyer/agent.rb:94:in `call'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/octokit-2.3.1/lib/octokit/client.rb:243:in `request'
	from /opt/boxen/repo/.bundle/ruby/2.0.0/gems/octokit-2.3.1/lib/octokit/client.rb:186:in `paginate'
	from /opt/boxen/repo/.b
```