Partially revert 6ecb30d, it breaks the release signing process

Author: shana

src/GitHub.VisualStudio/packages.config

@@ -29,7 +29,7 @@
   <package id="Microsoft.VisualStudio.Threading" version="14.1.131" targetFramework="net461" />
   <package id="Microsoft.VisualStudio.Utilities" version="14.3.25407" targetFramework="net461" />
   <package id="Microsoft.VisualStudio.Validation" version="14.1.111" targetFramework="net461" />
-  <package id="Microsoft.VSSDK.Vsixsigntool" version="15.0.26201" targetFramework="net461" />
+  <package id="Microsoft.VSSDK.Vsixsigntool" version="14.1.24720" targetFramework="net45" />
   <package id="Newtonsoft.Json" version="6.0.8" targetFramework="net45" />
   <package id="NLog" version="3.1.0" targetFramework="net45" />
   <package id="NullGuard.Fody" version="1.4.6" targetFramework="net461" developmentDependency="true" />