update to C# 7.1 so we can use all the latest features
update to C# 7.1 so we can use all the latest features

alinasmirnova/BenchmarkDotNet,ig-sinicyn/BenchmarkDotNet,Ky7m/BenchmarkDotNet,adamsitnik/BenchmarkDotNet,PerfDotNet/BenchmarkDotNet,ig-sinicyn/BenchmarkDotNet,adamsitnik/BenchmarkDotNet,Ky7m/BenchmarkDotNet,Ky7m/BenchmarkDotNet,alinasmirnova/BenchmarkDotNet,adamsitnik/BenchmarkDotNet,PerfDotNet/BenchmarkDotNet,ig-sinicyn/BenchmarkDotNet,Ky7m/BenchmarkDotNet,PerfDotNet/BenchmarkDotNet,alinasmirnova/BenchmarkDotNet,alinasmirnova/BenchmarkDotNet,ig-sinicyn/BenchmarkDotNet,adamsitnik/BenchmarkDotNet