Use password auth for ubuntu machines [#74]

It appears that having a password in database.yml, but not having one
in your postgres user, is fine. So we'll use password auth on ubuntu and
continue using passwordless (which works on Postgres.app installs) on
OSX.
Merge pull request #76 from AndrewLorente/ubuntu-postgres-setup

Use password auth for ubuntu machines [#74]
Merge branch 'master' of https://github.com/code-scouts/green_mercury into exception_notification
Update rake version; should fix travis
Merge branch 'master' into exception_notification