Update ruby version (#248)

Needed to work with newer Netlify images
Also, reduce clone to shallow

Fixes #238

Author: malfet

.ruby-version

@@ -1 +1 @@
-2.5.1
+2.6.8

preview_setup.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 rm -rf pytorch.github.io
-git clone --recursive https://github.com/pytorch/pytorch.github.io.git -b site
+git clone --recursive https://github.com/pytorch/pytorch.github.io.git -b site --depth 1
 cp *.md pytorch.github.io/_hub
 cp images/* pytorch.github.io/assets/images/