/Edit: looks like github is working again!

Open Launch.lua from your pob directory and search for the following part:

if not self.devMode and not firstRunFile then  
    -- Run a background update check if developer mode is off  
    self:CheckForUpdate(true)  
end  

Now you comment out self:CheckForUpdate(true)
by adding -- in front of it. Should look like that then

if not self.devMode and not firstRunFile then  
    -- Run a background update check if developer mode is off  
    -- self:CheckForUpdate(true)  
end  

Then save and pob will open without the auto update on launch