Add support for VS Team Explorer 2017

Author: jcansdale

src/GitHub.VisualStudio/source.extension.vsixmanifest

@@ -13,6 +13,7 @@
   </Metadata>
   <Installation AllUsers="true" Experimental="true">
     <InstallationTarget Id="Microsoft.VisualStudio.Community" Version="[14.0,15.0]" />
+    <InstallationTarget Version="[15.0,16.0)" Id="Microsoft.VisualStudio.IntegratedShell" />
   </Installation>
   <Dependencies>
     <Dependency Id="Microsoft.Framework.NDP" DisplayName="Microsoft .NET Framework" d:Source="Manual" Version="[4.5,)" />