Upgrade nuget package referenced in build script

Author: Jericho

build.cake

@@ -8,7 +8,7 @@
 #tool nuget:?package=GitVersion.CommandLine&version=5.2.4
 #tool nuget:?package=GitReleaseManager&version=0.11.0
 #tool nuget:?package=OpenCover&version=4.7.922
-#tool nuget:?package=ReportGenerator&version=4.5.3
+#tool nuget:?package=ReportGenerator&version=4.5.6
 #tool nuget:?package=coveralls.io&version=1.4.2
 #tool nuget:?package=xunit.runner.console&version=2.4.1