Update VSToolsPath to not be hard-coded to specific VS version

Author: Dustin Masters

src/React.Sample.Cassette/React.Sample.Cassette.csproj

@@ -208,7 +208,7 @@
   </ItemGroup>
   <PropertyGroup>
     <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">10.0</VisualStudioVersion>
-    <VSToolsPath Condition="'$(VSToolsPath)' == ''">C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\</VSToolsPath>
+    <VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
   </PropertyGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="$(VSToolsPath)\WebApplications\Microsoft.WebApplication.targets" Condition="'$(VSToolsPath)' != ''" />

src/React.Sample.Mvc4/React.Sample.Mvc4.csproj

@@ -190,7 +190,7 @@
   </ItemGroup>
   <PropertyGroup>
     <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">10.0</VisualStudioVersion>
-    <VSToolsPath Condition="'$(VSToolsPath)' == ''">C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\</VSToolsPath>
+    <VSToolsPath Condition="'$(VSToolsPath)' == ''">$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)</VSToolsPath>
   </PropertyGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="$(VSToolsPath)\WebApplications\Microsoft.WebApplication.targets" Condition="'$(VSToolsPath)' != ''" />
@@ -227,7 +227,7 @@
     </PropertyGroup>
     <Error Condition="!Exists('..\packages\JavaScriptEngineSwitcher.V8.Native.win-x64.3.0.0\build\JavaScriptEngineSwitcher.V8.Native.win-x64.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\JavaScriptEngineSwitcher.V8.Native.win-x64.3.0.0\build\JavaScriptEngineSwitcher.V8.Native.win-x64.props'))" />
   </Target>
-  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it.
        Other similar extension points exist, see Microsoft.Common.targets.
   <Target Name="BeforeBuild">
   </Target>