When defining a CoreOS channel in config.rb, the value should be quoted.

If not, the following error happens:

  There was an error loading a Vagrantfile. The file being loaded
  and the error message are shown below. This is usually caused by
  a syntax error.

  Path: .../coreos-vagrant/Vagrantfile
  Message: undefined local variable or method `beta' for main:Object"
 or
  Message: undefined local variable or method `alpha' for main:Object"