Remove database host config for Linux support

Created by: gracewashere

Fixes #296 (closed).

Problem:

Ubuntu (and possibly other Linux distributions) don't recognize localhost as a valid host parameter in config/database.yml.

Running bin/setup gives them the error:

Couldn't create database for {"adapter"=>"postgresql",
"database"=>"administrate-prototype_development",
"encoding"=>"utf8",
"host"=>"localhost",
"min_messages"=>"warning",
"pool"=>2,
"timeout"=>5000}

Solution:

Remove the host key from config/database.yml.

Without the host key, OS X defaults to localhost and Ubuntu defaults to its Linux-y equivalent.