Add slc6-gcc7 to PlatformInfo.cpp

• [2018-04-30 10:34] Validation started with lhcb-gaudi-merge#503
• [2018-05-01 00:09] Validation started with lhcb-head#1838
• [2018-05-01 00:11] Validation started with lhcb-clang-test#938
• [2018-05-01 00:11] Validation started with lhcb-gaudi-head#1853
• [2018-05-01 00:12] Validation started with lhcb-lcg-dev4#511
• [2018-05-01 00:12] Validation started with lhcb-gaudi-head-py3#140
• [2018-05-01 00:14] Validation started with lhcb-tdr-test#137
• [2018-05-01 00:15] Validation started with lhcb-lcg-dev3#511

merged

mentioned in commit 8afce0ea

mentioned in commit ece0cbe4

mentioned in merge request !1880 (merged)