Fix build with latest ROOT by switching to std::unordered_{set,map}.

• [2017-10-10 20:29] Validation started with lhcb-gaudi-head#725
• [2017-10-11 00:11] Validation started with lhcb-2017-patches#167
• [2017-10-11 00:18] Validation started with lhcb-gaudi-head-noavx#158
• [2017-10-11 00:18] Validation started with lhcb-clang-test#732
• [2017-10-11 00:19] Validation started with lhcb-head#1624
• [2017-10-11 00:19] Validation started with lhcb-gaudi-head-noavx2#139
• [2017-10-11 00:19] Validation started with lhcb-gaudi-head#1622

Thanks!

merged

mentioned in commit 8258a609

changed milestone to %v28r1