pythonPackages.sasmodels: fix build (#40381)

* The revision checksum changed, so it's safer to pin the package
  against an explicit `git` tag (https://github.com/NixOS/nixpkgs/pull/40381#issuecomment-388484695)

* Repaired `checkPhase` to run `py.test` properly on Python {2,3}

* Bumped to latest revision (unstable-2018-04-27) as it supports the entire test suite
  properly.