script/default-site: add `ruby` in exec command.
script/default-site: add `ruby` in exec command.

Windows doesn't recognize `exe/jekyll` as Ruby executable so this should work everywhere.

rlugojr/jekyll,ryanshaw/jekyll,x-way/jekyll,darwin/jekyll,yhironaka/yhironaka.github.io,ZDroid/jekyll,gencer/jekyll,Strangehill/jekyll,alex-kovac/jekyll,rlugojr/jekyll,alex-kovac/jekyll,darwin/jekyll,ZDroid/jekyll,x-way/jekyll,gencer/jekyll,alfredxing/jekyll,Strangehill/jekyll,ryanshaw/jekyll,x-way/jekyll,tomjohnson1492/jekyll,jaybe-jekyll/jekyll,rasa2011/rasa2011.github.io,gencer/jekyll,alex-kovac/jekyll,noikiy/jekyll,fulldecent/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,x-way/jekyll,ryanshaw/jekyll,alex-kovac/jekyll,rlugojr/jekyll,tillgrallert/jekyll,noikiy/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,darwin/jekyll,jekyll/jekyll,Strangehill/jekyll,Strangehill/jekyll,alfredxing/jekyll,getandpost/jekyll.github.io,chrisfinazzo/jekyll,xiebinhqy/jklly_china,chrisfinazzo/jekyll,fulldecent/jekyll,tillgrallert/jekyll,jekyll/jekyll,tomjohnson1492/jekyll,tillgrallert/jekyll,xiebinhqy/jklly_china,jekyll/jekyll,noikiy/jekyll,rlugojr/jekyll,jaybe-jekyll/jekyll,gencer/jekyll,rasa2011/rasa2011.github.io,gencer/jekyll,tomjohnson1492/jekyll,jekyll/jekyll,ZDroid/jekyll,jaybe-jekyll/jekyll,darwin/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,noikiy/jekyll,rasa2011/rasa2011.github.io,tomjohnson1492/jekyll,tomjohnson1492/jekyll,fulldecent/jekyll,alex-kovac/jekyll,Strangehill/jekyll,getandpost/jekyll.github.io,alfredxing/jekyll,xiebinhqy/jklly_china,chrisfinazzo/jekyll,xiebinhqy/jklly_china,chrisfinazzo/jekyll,alfredxing/jekyll