Use environment vars for nuget package versions

[skip ci]

Author: craigminihan

.appveyor.yml

@@ -2,15 +2,20 @@ platform: Any CPU
 image: Visual Studio 2017
 configuration: Debug
 
+environment:
+ OPENCOVER_VER: 4.6.519
+ XUNIT_VER: 2.2.0
+ COVERALLS_VER: 0.412
+
 before_build:
  - nuget restore
- - nuget install xunit.runner.console -Version 2.2.0 -OutputDirectory buildtools
- - nuget install OpenCover -Version 4.6.519 -OutputDirectory buildtools
- - nuget install coveralls.net -Version 0.412 -OutputDirectory buildtools
+ - cmd: nuget install xunit.runner.console -Version %XUNIT_VER% -OutputDirectory buildtools
+ - cmd: nuget install OpenCover -Version %OPENCOVER_VER% -OutputDirectory buildtools
+ - cmd: nuget install coveralls.net -Version %COVERALLS_VER% -OutputDirectory buildtools
 
 build_script:
  - cmd: msbuild /p:Configuration=%CONFIGURATION% HttpWebClient.sln
 
 test_script:
- - cmd: buildtools\OpenCover.4.6.519\tools\OpenCover.Console.exe -register:user -target:"buildtools\xunit.runner.console.2.2.0\tools\xunit.console.exe" -targetargs:"HttpWebClient.UnitTests\bin\%CONFIGURATION%\HttpWebClient.UnitTests.dll -noappdomain" -filter:"+[HttpWebClient]*"
+ - cmd: buildtools\OpenCover.%OPENCOVER_VER%\tools\OpenCover.Console.exe -register:user -target:"buildtools\xunit.runner.console.%XUNIT_VER%\tools\xunit.console.exe" -targetargs:"HttpWebClient.UnitTests\bin\%CONFIGURATION%\HttpWebClient.UnitTests.dll -noappdomain" -filter:"+[HttpWebClient]*"