Linux Ubuntu 17.04 64 bit, GTX 750
Blender v2.78.5 testbuild 2.79
Broken: No Principled BSDF option in ADD menu --> Shader
Worked: It is located in the properties window for the surface option of a material.
Select object- assign new material- open node editor- "Shift A" to add node, choose "Shader"- no "Principled BSDF"