Puppet users - Nov 2014 - Issue getting new custom fact to work in OS X

I'm trying to get a new custom fact working in OS X.  Source information is 
from:

https://github.com/grahamgilbert/grahamgilbert-mac_facts/blob/master/lib/facter/mac_admin_users.rb

Here is the process that I went through.

I added the .rb file to /etc/facter/facts.d/mac_admin_users.rb and modified 
the permissions to match existing custom facts.

-rwxr-xr-x  1 root  wheel   272 Nov 17 08:11 mac_admin_users.rb

When I run

sudo facter

I get the following error:

/etc/facter/facts.d/mac_admin_users.rb: line 3: syntax error near unexpected
token `:mac_admin_users'
/etc/facter/facts.d/mac_admin_users.rb: line 3: `Facter.add(:mac_admin_users)
do'
Fact file /etc/facter/facts.d/mac_admin_users.rb was parsed but returned an
empty data set

I tried to get more information with

sudo facter --debug

but that did not produce anything different.

Here is my file:

#mac_admin_users.rb
#require 'facter'
Facter.add(:mac_admin_users) do
  confine :kernel => "Darwin"
  setcode do
      admins = Facter::Util::Resolution.exec("dscl . -read /Groups/admin
GroupMembership")

      admins.gsub('GroupMembership: ','').gsub(' ',
', ')
  end
end

I also tested the dscl command on its own just to make sure I didn't have 
an error there but that worked.

I do have another custom fact running that outputs the time machine status 
and that is working.

I'm new to implementing custom facts and I really don't know where to 
start.  Is there something that I have missed in the syntax?


System Details:

OS X version 10.10

facter version: 2.3.0

MacBook Pro Retina

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