Fixed issue with switching function derivative in PairAIREBO::FLJ

Since we compute dvdw as d vdw / d rij, we have to also compute
dslw as d slw / d rij. Currently, we compute -1/r d slw/d rij,
which leads to incorrect results when the two are later combined.
Alternatively, one could also modify dvdw to be -1/r d vdw/d rij,
which would be a more standard way to do LJ calculations, but this
way seems more consistent.