How to use an old Macbook Pro as a server with lid closed and preventing sleep ?

I am looking to use an old Macbook Pro as a server with Yosemite Server.

I want the Macbook Pro to be able to close the lid (for space purposes) while preventing the mac from going to sleep.

In short, I want to be able to use it as a headless server.

Is there a simple way to do this ? Or is Insomniax the only way to go ?

Thanks,

Daniel