define [], () ->
 defaultRepo:
   repoUser: 'oerpub'
   repoName: 'textbook-demo'
   branch: 'master'

define [], () ->
 defaultRepo:
   repoUser: 'oerpub'
   repoName: 'textbook-demo'
   branch: ''

Set the branch to '', that makes it use the default.
Set the branch to '', that makes it use the default.

It also means the repo in the url does not by default contain /branch/master.
