Update shift_wrappers.py (#1579)
Update shift_wrappers.py (#1579)

Added the flag `-para old`  to ppm package command to adapt it to the newer versions of ppm.
gph82/mdtraj,rmcgibbo/mdtraj,dwhswenson/mdtraj,gph82/mdtraj,gph82/mdtraj,dwhswenson/mdtraj,mdtraj/mdtraj,rmcgibbo/mdtraj,mdtraj/mdtraj,dwhswenson/mdtraj,mdtraj/mdtraj,rmcgibbo/mdtraj