upgrade tool for C# coverage reports

src/csharp/.nuget/packages.config

 <?xml version="1.0" encoding="utf-8"?>
 <packages>
   <package id="NUnit.Runners" version="2.6.4" />
-  <package id="OpenCover" version="4.6.166" />
-  <package id="ReportGenerator" version="2.3.2.0" />
+  <package id="OpenCover" version="4.6.519" />
+  <package id="ReportGenerator" version="2.4.4.0" />
 </packages>
\ No newline at end of file

tools/run_tests/run_csharp.bat

@@ -10,9 +10,9 @@ if not "%CONFIG%" == "gcov" (
 ) else (
   @rem Run all tests with code coverage
 
-  packages\OpenCover.4.6.166\tools\OpenCover.Console.exe -target:"packages\NUnit.Runners.2.6.4\tools\nunit-console-x86.exe" -targetdir:"." -targetargs:"%*" -filter:"+[Grpc.Core]*"  -register:user -output:coverage_results.xml || goto :error
+  packages\OpenCover.4.6.519\tools\OpenCover.Console.exe -target:"packages\NUnit.Runners.2.6.4\tools\nunit-console-x86.exe" -targetdir:"." -targetargs:"%*" -filter:"+[Grpc.Core]*"  -register:user -output:coverage_results.xml || goto :error
 
-  packages\ReportGenerator.2.3.2.0\tools\ReportGenerator.exe -reports:"coverage_results.xml" -targetdir:"..\..\reports\csharp_coverage" -reporttypes:"Html;TextSummary" || goto :error
+  packages\ReportGenerator.2.4.4.0\tools\ReportGenerator.exe -reports:"coverage_results.xml" -targetdir:"..\..\reports\csharp_coverage" -reporttypes:"Html;TextSummary" || goto :error
 
   @rem Generate the index.html file
   echo ^<html^>^<head^>^</head^>^<body^>^<a href='csharp_coverage/index.htm'^>csharp coverage^</a^>^<br/^>^</body^>^</html^> >..\..\reports\index.html