Upgrade asmjit

Updated lepton in order to work with the following versions on asmjit:
3fcd65cf805dcd4bbd5fe89a11993dabd01d7b3d
Mon Dec 7 07:34:25 2015
to
fb9f82cb61df36aa513d054e748dc6769045f33e
Fri Nov 11 22:34:49 2016

NOTICE THAT AS OF NOW THE LEPTON LIBRARY INCLUDED IN PLUMED WILL BE
MODIFIED WRT THE ONE INCLUDED IN OPENMM