Added PackageId to vsixmanifest.

Author: grokys

src/GitHub.VisualStudio/source.extension.vsixmanifest

@@ -4,6 +4,7 @@
     <Identity Id="c3d3dc68-c977-411f-b3e8-03b0dccf7dfc" Version="2.1.1.3" Language="en-US" Publisher="GitHub, Inc" />
     <DisplayName>GitHub Extension for Visual Studio</DisplayName>
     <Description xml:space="preserve">A Visual Studio Extension that brings the GitHub Flow into Visual Studio.</Description>
+    <PackageId>GitHub.VisualStudio</PackageId>
     <MoreInfo>https://visualstudio.github.com</MoreInfo>
     <License>LICENSE.txt</License>
     <Icon>Resources\[email protected]</Icon>