Remove pin to mdtraj version 1.5.1
Remove pin to mdtraj version 1.5.1

Most recent MDTraj release should obviate it.

dwhswenson/openpathsampling,openpathsampling/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,dwhswenson/openpathsampling,jhprinz/openpathsampling,openpathsampling/openpathsampling,jhprinz/openpathsampling,dwhswenson/openpathsampling,dwhswenson/openpathsampling,jhprinz/openpathsampling,choderalab/openpathsampling