RF: use explicit sort key for sympy sorting
RF: use explicit sort key for sympy sorting

Sympy > 0.7.3 raises an error attempting to sort sympy expressions
without a key, but provides ``default_sort_key`` for backwards
compatibility.

arokem/nipy,alexis-roche/niseg,alexis-roche/nipy,arokem/nipy,alexis-roche/niseg,bthirion/nipy,nipy/nireg,nipy/nireg,alexis-roche/register,arokem/nipy,arokem/nipy,bthirion/nipy,alexis-roche/nipy,alexis-roche/nipy,alexis-roche/nireg,bthirion/nipy,bthirion/nipy,alexis-roche/register,alexis-roche/nipy,alexis-roche/nireg,alexis-roche/register