Install crontab w/ script

This adds a couple of new scripts: install-crontab.sh and update.sh. The
install-crontab script does exactly that -- it installs a new crontab
that runs the update script every 5 minutes. An update script was added
to simplify the crontab a bit and to ensure the Elixir update script is
executed from the correct directory.

update.exs

@@ -6,7 +6,7 @@ defmodule Instances do
 
   def init() do
     File.rename(@update_file, "#{@update_file}-prev")
-    update
+    update()
   end
 
   def request(url) do