Bump Git version to 2.13.6

doc/install/installation.md

@@ -80,7 +80,7 @@ Make sure you have the right version of Git installed
     # Install Git
     sudo apt-get install -y git-core
-    # Make sure Git is version 2.13.0 or higher
+    # Make sure Git is version 2.13.6 or higher
     git --version
 Is the system packaged Git too old? Remove it and compile from source.