These are reference values when using a BoxMesh (when students
don't have internet access), see demo beam.py.

Reference value 2015-08-06 (FEniCS 1.5+)
Average z-displacement: -0.6052757910100011   (standard formulation)

Reference value 2015-08-06 (FEniCS 1.3)
Average z-displacement: -0.6052757910100011   (standard formulation)