don't nuke CNAME when updating gh-pages branch

deploy_ghpages.sh

@@ -28,7 +28,7 @@ rm -Rf gh-pages
 git clone -b gh-pages --single-branch https://${GH_TOKEN}@github.com/${repo} gh-pages > /dev/null
 cd gh-pages
 
-rsync -az --stats --delete --exclude .git --force ../build/asciidoc/html5/ .
+rsync -az --stats --delete --exclude .git --exclude CNAME --force ../build/asciidoc/html5/ .
 
 git config --local user.email "[email protected]"
 git config --local user.name "Travis"