Need test job against older SymPy version

My desktop computer runs Ubuntu 18.04, which includes SymPy 1.1.1. pystencils generally works fine on that version, but it is sufficiently different that we have occasionally broken support of it in the past, e.g. !105 (merged). Furthermore, a slightly different simplification engine in newer Sympy versions has previously masked actual bugs (lbmpy!14 (closed) / 721fdf45). To solve this properly, we should define a minimum SymPy version and perform CI tests using that specific version. Currently we only test the latest release and the latest master.