[WIP] Fix to ppm chemical shift wrapper, better tests, CI integration.

shiftx2 is written in Python, and presumably does not work in Python 3 (I've never tried).

If mdtraj is being invoked with Python 3, it's safest to assume that's the default (and potentially _only_) interpreter available, even if in some cases it can be coerced to work.

An alternative would be to change the shiftx2 command to: `"python2 %s -b 'trj*.pdb'" % binary` rather than the current version.