Adapt to change of free_symbols in sympy 1.2

[goerz]

SymPy made a backwards-incompatible change to how free_symbols works for Indexed expressions. See https://github.com/sympy/sympy/pull/13360.

Currently, on Travis and AppVeyor, we enforce SymPy < 1.2 to avoid this problem. We should remove this restriction by investigating what the consequences SymPy's change are for QNET, and whether we need additional tests. Anything that relies on free_symbols needs to be checked carefully -- especially derivatives.

Tests should be written in such a way that they pass both for SymPy < 1.2 and SymPy >= 1.2.