License for Continuous Integration Server

I'm a freelancer developing a plugin using Unity 5.x Personal. I have a potential customer for the plugin but we are confused about a licensing issue.

I build releases of my Unity plugin on a dedicated machine which is used as a continuous integration server. This seems to be covered by the Personal License so long as I don't make more than $100,000 from my Unity plugin.

The customer has one license for Unity 5.x Pro which is used by their sole Unity developer. They want to build the plugin on their own Continuous Integration server which is shared by several developers including the single Unity developer. No actual development on the server.

Does a Pro License cover this situation? If not, what license do they need? Thanks!