How can I get a list of the roles in a system, via the REST API? Similar to How to Retrieve all Forums, how can I retrieve the list of roles in a system via the Services REST API? 

This is Drupal 7, Services 3.x. 

Are roles terms in a taxonomy vocabulary ?
Create a custom service resource, (you'll have to create your own access callback and access callback file, unless you want to use DrupalGap which is tailored to the link mentioned here)
Then in your callback, try something like this:

return user_roles();