Fixed gcc7 issue with yaml-cpp 0.6.3
Fixed gcc7 issue with yaml-cpp 0.6.3

vmamidi/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,vmamidi/trafficserver,pbchou/trafficserver,duke8253/trafficserver,duke8253/trafficserver,pbchou/trafficserver,bryancall/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,vmamidi/trafficserver,bryancall/trafficserver,bryancall/trafficserver,pbchou/trafficserver,bryancall/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,duke8253/trafficserver,duke8253/trafficserver,duke8253/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver