Added "ServiceFabricBackup/"

The latest update of Visual Studio 17 (15.5.5) introduces a new backup folder called "ServiceFabricBackup" before upgrading service fabric applications.

VisualStudio.gitignore

@@ -237,6 +237,7 @@ _UpgradeReport_Files/
 Backup*/
 UpgradeLog*.XML
 UpgradeLog*.htm
+ServiceFabricBackup/
 
 # SQL Server files
 *.mdf