Step up DC-DC converter for powering Raspberry Pi from batteries
That part they used is only guaranteed to operate down to 5.0V, which is actually worse than the Raspberry PI. Maybe some of them will work okay, maybe they won't. 



Something based on the LM2577 should work. It actually cuts off at a bit too low a voltage to protect the batteries, so I'd add something external to shut it down at about 4.0V pack voltage. You don't gain much run time from running the batteries down to 0.725V/cell, and series-connected batteries can be damaged.