Use correct database.yml property for host name

From the documentation I can find, the correct property name here is actually "host", not "hostname" (see http://guides.rubyonrails.org/configuring.html#configuring-a-database). I deployed this gem on a hosting provider that had a separate database instance and it blew up on me because they also use "host" in the "database.yml". Changing this to match what seems to be the correct Rails name.