Added optimisation in clique_covering_number: use matching number for triangle free graphs

Old version:
sage: dodec = graphs.DodecahedralGraph()
sage: bucky = graphs.BuckyBall()
sage: %timeit clique_covering_number(dodec)
1 loops, best of 3: 1.21 s per loop
sage: clique_covering_number(bucky)
(Killed after some time)

New version:
sage: dodec = graphs.DodecahedralGraph()
sage: bucky = graphs.BuckyBall()
sage: %timeit clique_covering_number(dodec)
100 loops, best of 3: 2.34 ms per loop
sage: %timeit clique_covering_number(bucky)
100 loops, best of 3: 16 ms per loop