test-parallel-ug-mpi-2 fails with clang

When compiled with clang-3.8 and libstdc++-6.2 (Ubuntu 16.10) the test test-parallel-ug fails for 2 mpi ranks with the error

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mpirun noticed that process rank 1 with PID 12585 on node patton exited on signal 4 (Illegal instruction).
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Notice that this is not for master but for core/dune-grid!101 (merged) because UG does not work with clang otherwise.