Package System.ValueTuple, v4.0.3.0 with VSIX

System.ValueTuple isn't included with Visual Studio 2015 so we must
include it in the .vsix.
Link it from NuGet folder rather than include binary in project.

Author: jcansdale

src/GitHub.VisualStudio/GitHub.VisualStudio.csproj

@@ -414,6 +414,11 @@
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       <IncludeInVSIX>true</IncludeInVSIX>
     </Content>
+    <Content Include="..\..\packages\System.ValueTuple.4.5.0\lib\net461\System.ValueTuple.dll">
+      <Link>System.ValueTuple.dll</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+      <IncludeInVSIX>true</IncludeInVSIX>
+    </Content>
     <None Include="packages.config">
       <SubType>Designer</SubType>
     </None>

src/GitHub.VisualStudio/source.extension.vsixmanifest

@@ -40,6 +40,7 @@
     <Asset Type="Microsoft.VisualStudio.MefComponent" d:Source="Project" d:ProjectName="GitHub.VisualStudio.UI" Path="|GitHub.VisualStudio.UI|" />
     <Asset Type="Microsoft.VisualStudio.MefComponent" d:Source="File" Path="Rothko.dll" />
     <Asset Type="Microsoft.VisualStudio.Assembly" d:Source="File" Path="Newtonsoft.Json.dll" AssemblyName="Newtonsoft.Json, Version=9.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" />
+    <Asset Type="Microsoft.VisualStudio.Assembly" d:Source="File" Path="System.ValueTuple.dll" AssemblyName="System.ValueTuple, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
   </Assets>
   <Prerequisites>
     <Prerequisite Id="Microsoft.VisualStudio.Component.CoreEditor" Version="[15.0.25824.0,)" DisplayName="Visual Studio core editor" />