Use C# 7.1 to fix CS8306 error when building JsonRpc.csproj.

Author: tintoy

src/JsonRpc/JsonRpc.csproj

@@ -2,6 +2,7 @@
     <PropertyGroup>
         <TargetFrameworks>net46;netstandard1.6</TargetFrameworks>
         <PlatformTarget>AnyCPU</PlatformTarget>
+        <LangVersion>Latest</LangVersion>
         <!-- Needed for Microsoft.Composition -->
         <PackageTargetFallback Condition=" '$(TargetFramework)' == 'netstandard1.6' ">$(PackageTargetFallback);dotnet5.4;portable-net45+win8</PackageTargetFallback>
         <PackageVersion Condition="'$(GitVersion_NuGetVersion)' != ''">$(GitVersion_NuGetVersion)</PackageVersion>